CHAPTER XVII - STRUCTURE OF THE NERVOUS SYSTEM
Coördination and Adjustment.—If we consider for a moment the movements of the body, we cannot fail to note the coöperation of organs, one with another. In the simple act of whittling a stick one hand holds the stick and the other the knife, while the movements of each hand are such as to aid in the whittling process. Examples of coöperation are also found in the taking of food, in walking, and in the performance of different kinds of work. Not only is coöperation found among the external organs, but our study of the vital processes has shown that the principle of coöperation is carried out by the internal organs as well. The fact that all the activities of the body are directed toward a common purpose makes the coöperation of its parts a necessity. The term "coördination" is employed to express this coöperation, or working together, of the different parts of the body.
A further study of the movements of the body shows that many of them have particular reference to things outside of it. In going about one naturally avoids obstructions, and if anything is in the way he walks around or steps over it. Somewhat as a delicate instrument (the microscope for example) is altered or adjusted, in order to adapt it to its work, the parts of the body, and the body as a whole, have to be adjusted to their surroundings. This is seen in the attitude assumed in sitting and in standing, in the position of the hands for different kinds[pg 280] of work, in the variations of the circulation of the blood in the skin, and in the movements for protecting the body.[95]
Work of the Nervous System.—How are the different activities of the body controlled and coördinated? How is the body adjusted to its surroundings? The answer is found in the study of the nervous system. Briefly speaking, the nervous system controls, coördinates, and adjusts the different parts of the body by fulfilling two conditions:
1. It provides a complete system of connections throughout the body, thereby bringing all parts into communication.
2. It supplies a means of controlling action (the so-called impulse) which it passes along the nervous connections from one part of the body to another.
The present chapter deals with the first of these conditions; the chapter following, with the second.
The Nerve Skeleton.—If all the other tissues are removed, leaving only the nervous tissue, a complete skeleton outline of the body still remains. This nerve skeleton, as it has been called, has the general form of the framework of bones, but differs from it greatly in the fineness of its structures and the extent to which it represents every portion of the body. An examination of a nerve skeleton, or a diagram of one (Fig. 125), shows the main structures of the nervous system and their connection with the different parts of the body.
Corresponding to the skull and the spinal column is a central nervous axis, made up of two parts, the brain and the spinal cord. From this central axis white, cord-like bodies emerge and pass to different parts of the body. [pg 281]These are called nerve trunks, and the smaller branches into which they divide are called nerves. The nerves also undergo division until they terminate as fine thread-like structures in all parts of the body. The distribution of nerve terminations, however, is not uniform, as might be supposed, but the skin and important organs like the heart, stomach, and muscles are the more abundantly supplied. On many of the nerves are small rounded masses, called ganglia, and from many of these small nerves also emerge. At certain places the nerves and ganglia are so numerous as to form a kind of network, known as a plexus.
Fig. 125—Diagram of nerve skeleton. The illustration shows the extent and general arrangement of the nervous tissue. A. Brain. B. Spinal cord. N. Nerve trunks and nerves. G. Ganglia.
It is through these structures—brain and spinal cord, nerve trunks and nerves, ganglia and nerve terminations—that connections are established between all parts of the body, but more especially between the surface of the body and the organs within.
The Neurons, or Nerve Cells.—While a hasty examination of the nerve skeleton is sufficient to show the connection[pg 282] of the nervous system with all parts of the body, no amount of study of its gross structures reveals the nature of its connections or suggests its method of operation. Insight into the real nature of the nervous system is obtained only through a study of its minute structural elements. These, instead of being called cells, as in the case of the other tissues, are called neurons. The use of this term, instead of the simpler one of nerve cell, is the result of recent advances in our knowledge of the nervous system.[96]
Fig. 126—Diagram of a mon-axonic neuron (greatly enlarged except as to length). The central thread in the axon is the axis cylinder.
The neurons are in all respects cells. They differ widely, however, from all the other cells of the body and are, in some respects, the most remarkable of all cells. They are characterized by minute extensions, or prolongations, which in some instances extend to great distances. Though the neurons in certain parts of the body differ greatly in form and size from those in other parts of the body, most of them may be included in one or the other of two classes, known as mon-axonic neurons and di-axonic neurons.
Mon-axonic Neurons.—Neurons of this[pg 283] class consist of three distinct parts, known as the cell-body, the dendrites, and the axon (Fig. 126).
The cell-body has in itself the form of a complete cell and was at one time so described. It consists of a rounded mass of protoplasm, containing a well-defined nucleus. The protoplasm is similar to that of other cells, but is characterized by the presence of many small granules and has a slightly grayish color.
The dendrites are short extensions from the cell-body. They branch somewhat as the roots of a tree and form in many instances a complex network of tiny rootlets. Their protoplasm, like that of the cell-body, is more or less granular. The dendrites increase greatly the surface of the cell-body, to which they are related in function.
The axon, or nerve fiber, is a long, slender extension from the cell-body, which connects with some organ or tissue. It was at one time described as a distinct nervous element, but later study has shown it to be an outgrowth from the cell-body. The mon-axonic neurons are so called from their having but a single axon.
Di-axonic Neurons.—Neurons belonging to this class have each a well-defined cell-body and two axons, but no parts just like the dendrites of mon-axonic neurons. The cell-body is smooth and rounded, and its axons extend from it in opposite directions (Fig. 127).
Fig. 127—Diagram of a di-axonic neuron. The diagram shows only the conducting portion of the axon, or axis cylinder.
Structure of the Axon.—The axon, or nerve fiber, has practically the same structure in both classes of neurons, being composed in most cases of three distinct parts. In[pg 284] the center, and running the entire length of the axon, is a thread-like body, called the axis cylinder (Fig. 126). The axis cylinder is present in all axons and is the part essential to their work. It may be considered as an extension of the protoplasm from the cell-body. Surrounding the axis cylinder is a thick, whitish-looking layer, known as the medullary sheath, and around this is a thin covering, called the primitive sheath, or neurilemma. The medullary sheath and the primitive sheath are not, strictly speaking, parts of the nerve cell, but appear to be growths that have formed around it. Certain of the axons have no primitive sheath and others are without a medullary sheath.[97]
Form and Length of Axons.—Where the axons terminate they usually separate into a number of small divisions, thereby increasing the number of their connections. Certain axons are also observed to give off branches before the place of termination is reached (Fig. 131). These collateral branches, by distributing themselves in a manner similar to the main fiber, greatly extend the influence of a single neuron.
In the matter of length, great variation is found among the axons in different parts of the body. In certain parts of the brain, for example, are fibers not more than one one-hundredth of an inch in length, while the axons that pass all the way from the spinal cord to the toes have a length of more than three feet. Between these extremes practically all variations in length are found.
Arrangements of the Neurons.—Nowhere in the body do the neurons exist singly, but they are everywhere connected with each other to form the different structures observed in the nerve skeleton. Two general plans of connection are to be observed, known as the anatomical and the physiological, or, more simply speaking, as the "side-by-side" and "end-to-end" plans. The side-by-side[pg 285] plan is seen in that disposition of the neurons which enables them to form the nerves and the ganglia, as well as the brain and spinal cord. The end-to-end connections are necessary to the work which the neurons do.
Side-by-side Connections.—On separating the ganglia and nerves into their finest divisions, it is found that the nerves consist of axons, while the ganglia are made up mainly of cell-bodies and dendrites. The axons lie side by side in the nerve, being surrounded by the same protective coverings, while the cell-bodies form a rounded mass or cluster, which is the ganglion (Fig. 128). But the axons, in order to connect with the cell-bodies, must terminate within the ganglion, so that they too form a part of it. To some extent, also, axons pass through ganglia with which they make no connection. The neurons in the brain and spinal cord also lie side by side, but their arrangement is more complex than that in the nerves and ganglia.
Fig. 128—Diagrams illustrating arrangement of neurons. A, B. Ganglia and short segments of nerves. 1. Ganglion. 2. Nerve. In the ganglion of A are end-to-end connections of different neurons; in the ganglion of B are the cell-bodies of di-axonic neurons. C. Section of a nerve trunk. 1. Epineurium consisting chiefly of connective tissue. 2. Bundles of nerve fibers. 3. Covering of fiber bundle, or perineurium. 4. Small artery and vein.
[pg 286]The side-by-side arrangement of the neurons shows clearly the structure of the ganglia and nerves. The nerve is seen to be a bundle of axons, or nerve fibers, held together by connective tissue, while the ganglion is little more than a cluster of cell-bodies. Their connection is necessarily very close, for the same group of neurons will form, with their axons, the nerve, and, with their cell-bodies, the ganglion (Fig. 128).
End-to-end Connections.—These consist of loose end-to-end unions of the fiber branches of certain neurons with the dendrites of other neurons. The purpose of such connections is to provide the means of communication between different parts of the body. There appears to be no actual uniting of the fiber branches with the dendrites, but they come into relations sufficiently close to establish conduction pathways, and these extend throughout the body (Fig. 129). They connect all parts of the body with the brain and spinal cord, while connections within the brain and cord bring the parts into communication with each other.
Fig. 129—Diagram of a nerve path starting at the skin, extending through the spinal cord, and passing out to muscles. A division of this path also reaches the brain.
[pg 287]Nature of the Nervous System.—The nervous system represents the sum total of the neurons in the body. In some respects it may be compared to the modern telephone system. The neurons, like the electric wires, connect different places with a central station (the brain and spinal cord), and through the central station connections are established between the different places in the system. As the separate wires are massed together to form cables, the neurons are massed to form the gross structures of the nervous system. The nervous system, however, is so radically different from anything found outside of the animal body that no comparison can give an adequate idea of it. We now pass to a study of the gross structures observed in the nerve skeleton.
Divisions of the Nervous System.—While all of the nervous structures are very closely blended, forming one complete system for the entire body, this system presents different divisions which may, for convenience, be studied separately. As physiologists have become better acquainted with the human nervous system, different schemes of classification have been proposed. The following outline, based upon the location of the different parts, presents perhaps the simplest view of the entire group of nervous structures:
[pg 288]The Central Division.—This division of the nervous system lies within the cranial and spinal cavities, and consists of the brain and the spinal cord. The brain occupying the cranial cavity and the spinal cord in the spinal cavity connect with each other through the large opening at the base of the skull to form one continuous structure. The brain and cord are the most complicated portions of the nervous system, and the ones most difficult to understand.
Fig. 130—Diagram of divisions of brain.
The Brain.—The brain, which is the largest mass of nervous tissue in the body, weighs in the average sized man about 50 ounces, and in the average sized woman about 44 ounces.[98] It may be roughly divided into three parts, which are named from their positions (in lower animals) the forebrain, the midbrain, and the hindbrain (Fig. 130). The forebrain consists almost entirely of a single part, known as
The Cerebrum.—The cerebrum comprises about seven eighths of the entire brain, and occupies all the front, middle, back, and upper portions of the cranial cavity, spreading over and concealing, to a large extent, the parts beneath. The surface layer of the cerebrum is called the cortex. This is made up largely of cell-bodies, and has a grayish appearance.[99] The cortex is greatly increased in[pg 289] area by the presence everywhere of ridge-like convolutions, between which are deep but narrow depressions, called fissures. The interior of the cerebrum consists mainly of nerve fibers, or axons, which give it a whitish appearance. These fibers connect with the cell-bodies in the cortex (Fig. 131).
The cerebrum is a double organ, consisting of two similar divisions, called the cerebral hemispheres. These are separated by a deep groove, extending from the front to the back of the brain, known as the median fissure. The hemispheres, however, are closely connected by a great band of underlying nerve fibers, called the corpus callosum.
Fig. 131—Microscope drawing of a neuron from cerebral cortex. a. Short segment of the axis cylinder with collateral branches.
At the base of the cerebrum three large masses of cell-bodies are to be found. One of these, a double mass, occupies a central position between the hemispheres, and is called the optic thalami. The other two occupy front central positions at the base of either hemisphere, and are known as the corpora striata, or the striate bodies.
The Midbrain is a short, rounded, and compact body that lies immediately beneath the cerebrum, and connects[pg 290] it with the hindbrain. On account of the great size of the cerebrum, the midbrain is entirely concealed from view when the other parts occupy their normal positions. However, if the cerebrum is pulled away from the hindbrain, it is brought into view somewhat as in Fig. 130.
The midbrain carries upon its back and upper surface four small rounded masses of cell-bodies, called the corpora quadrigemina. The upper two of these bodies are connected with the eyes; the lower two appear to have some connection with the organs of hearing. On the front and under surface, the midbrain separates slightly as if to form two pillars, which are called the crura cerebri, or cerebral peduncles. These contain the great bundles of nerve fibers that connect the cerebrum with the parts of the nervous system below.
The Hindbrain lies beneath the back portion of the cerebrum, and occupies the enlargement at the base of the skull. It forms about one eighth of the entire brain, and is composed of three parts—the cerebellum, the pons, and the bulb.
The Cerebellum is a flat and somewhat triangular structure with its upper surface fitting into the triangular under surface of the back of the cerebrum. It is divided into three lobes—a central lobe and two lateral lobes—and weighs about two and one half ounces. In its general form and appearance, as well as in the arrangement of its cell-bodies and axons, the cerebellum resembles the cerebrum. It differs from the cerebrum, however, in being more compact, and in having its surface covered with narrow, transverse ridges instead of the irregular and broader convolutions (Fig. 132).
The Pons, or pons Varolii, named from its supposed resemblance to a bridge, is situated in front of the cerebellum, and is readily recognized as a circular expansion which extends forward from that body. It consists largely of[pg 291] bands of nerve fibers, between which are several small masses of cell-bodies. The fibers connect with different parts of the cerebellum and with parts above.
Fig. 132—Human brain viewed from below. C. Cerebrum. Cb. Cerebellum. M. Midbrain. P. Pons. B. Bulb. I-XII. Cranial nerves.
The Bulb, or medulla oblongata, is, properly speaking, an enlargement of the spinal cord within the cranial cavity. It is somewhat triangular in shape, and lies immediately below the cerebellum. It contains important clusters of cell-bodies, as well as the nerve fibers that pass from the spinal cord to the brain.
[pg 292]The Spinal Cord.—This division of the central nervous system is about seventeen inches in length and two thirds of an inch in diameter. It does not extend the entire length of the spinal cavity, as might be supposed, but terminates at the lower margin of the first lumbar vertebra.[100] It connects at the upper end with the bulb, and terminates at the lower extremity in a number of large nerve roots, which are continuous with the nerves of the hips and legs (Fig. 133). Two deep fissures, one in front and the other at the back, extend the entire length of the cord, and separate it into two similar divisions. These are connected, however, along their entire length by a central band consisting of both gray and white matter.
Fig. 133—Spinal cord, showing on one side the nerves and ganglia with which it is closely related in function. A. Bulb. B. Cervical enlargement. C. Lumbar enlargement. D. Termination of cord. E. Nerve roots that occupy the spinal cavity below the cord. P. Pons. D.G. Dorsal root ganglia. S.G. Sympathetic ganglia. N. Nerve trunks to upper and lower extremities.
The arrangement of the neurons of the spinal cord is just the reverse of[pg 293] that in the cerebrum—the center being occupied by a double column of cell-bodies, which give it a grayish appearance, while the fibers occupy the outer portion of the cord, giving it a whitish appearance.
The spinal cord is not uniform in thickness, but tapers slightly, though not uniformly, from the upper toward the lower end. At the places where the nerves from the arms and legs enter the cord two enlargements are to be found, the upper being called the cervical and the lower the lumbar enlargement. These, on account of the difference in length between the cord and the spinal cavity, are above—the lower one considerably above—the places where the limbs which they supply join the trunk (Fig. 133).
Arrangement of the Neurons of the Brain and Cord.—The cell-bodies in the brain and spinal cord are collected into groups, and their fibers extend from these groups to places that may be near or remote. Guided by the white and gray colors of the nervous tissue, and also by the structures revealed by the microscope, physiologists have made out three general schemes in the grouping of cell-bodies, as follows:
1. That of surface distribution, the cell-bodies forming a thin but continuous layer over a given surface. This is the plan in the cerebrum and cerebellum, and here are found devices for increasing the surface: the cerebrum having convolutions, the cerebellum transverse ridges.
2. That of collections of cell-bodies into rounded masses. Such masses are found in the bulb, the pons, the midbrain, and the base of the cerebrum.
3. That of arrangement in a continuous column. This is the plan in the spinal cord. It matters not at what place the spinal cord be cut, a central area of gray matter, resembling in form the capital letter H, is always found.
The fibers connecting with the cell-bodies in the brain and spinal cord are gathered into bundles or tracts, and these pass through different parts somewhat as follows:
1. In the cerebrum they extend in three general directions, forming three classes of fibers. The first connect different localities in the same hemisphere, and are known as association fibers (A, Fig. 134). The second make connection between the two hemispheres, and form[pg 294] the corpus callosum. These are known as commissural fibers (C, Fig. 134). The third connect the cerebrum with the parts of the nervous system below, and are called projection fibers (P, Fig. 134).
2. In the cerebellum both association and commissural fibers are found. Bands of fibers, passing upward toward the cerebrum and downward toward the cord, connect this part of the brain with other parts of the nervous system.
Fig. 134—Semi-diagrammatic representation of a section through the right cerebral hemisphere, showing fiber tracts. A. Association fibers. C. Commissural fibers. P. Projection fibers. The cell-bodies with which the fiber bundles connect are in the surface layer or cortex.
3. In the midbrain, bulb, and spinal cord fibers are found: first, that connect these parts with the cerebrum[101] and cerebellum above; second,[pg 295] that pass into and become a part of the nerves of the body; and third, that connect the opposite sides of these parts together.
The Peripheral Division.—The peripheral division of the nervous system includes all the nervous structures found outside of the brain and spinal cord. These consist of the cranial, spinal, and sympathetic nerves, and of various small ganglia, all of which are closely connected with the central system.
Spinal Nerves and Dorsal-root Ganglia.—The spinal nerves comprise a group of thirty-one pairs, which connect the spinal cord with different parts of the trunk, with the upper, and with the lower extremities. Each nerve joins the cord by two roots, these being named from their positions the ventral, or anterior, root and the dorsal, or posterior, root. The two roots blend together within the spinal cavity to form a single nerve trunk, which passes out between the vertebræ. On the dorsal root of each spinal nerve is a small ganglion which is named, from its position, the dorsal-root ganglion. (Consult Figs. 133 and 135, and also Fig. 125.)
Double Nature of Spinal Nerves.—Charles Bell, in 1811, made the remarkable discovery that each spinal nerve is double in function. He found the portion connecting with the cord by the dorsal root to be concerned in the production of feeling and the portion connecting by the ventral root to be concerned in the production of motion. In keeping with these functions, the two divisions of the nerve are made up of different kinds of fibers, as follows:
1. The dorsal-root divisions, of the fibers of di-axonic neurons, the cell-bodies of which form the dorsal-root ganglia (Fig. 135).
2. The ventral-root divisions, of the fibers of mon-axonic[pg 296] neurons, the cell-bodies of which are in the gray matter of the cord.
The first convey impulses to the cord and are called afferent neurons;[102] the second convey impulses from the cord and are known as efferent neurons. Thus, by forming a part of the nerve pathways between the skin and the brain, the dorsal divisions of these nerves aid in the production of feeling; and by completing pathways to the muscles, the ventral divisions aid in the production of motion (Figs. 129, 135, and 141).
Fig. 135—Connection of spinal nerves with the cord. On the right is shown a nerve pathway from the skin to the muscle. A division of this pathway reaches the brain.
The Cranial Nerves.—From the under front surface of the brain, twelve pairs of nerves emerge and pass to the head, neck, and upper portions of the trunk. These, the cranial nerves, have names suggestive of their function or distribution and, in addition, are given numbers which indicate the order in which they leave the brain (Fig. 136). Unlike the spinal nerves, the cranial nerves present great variety among themselves, scarcely any two of them being alike in function or in their connection with different parts of the body. Several of them have to do with the special senses, and are for this reason very important. They[pg 297] connect the brain with the different parts of the head, neck, and trunk, as follows:
1. The first pair (olfactory nerves; nerves of smell; afferent) connect with the mucous membrane of the nostrils (Fig. 136).
2. The second pair (optic nerves; nerves of sight; afferent) connect with the retina of the eyes.
3. The third, fourth, and sixth pairs (motores oculi; control muscles of the eyes; efferent) connect with the internal and external muscles of the eyeballs (Fig. 136).
Fig. 136—Diagram suggesting the distribution and functions of the cranial nerves (Colton). See also Fig. 132.
4. The fifth pair (trigeminal nerves; nerves of feeling[pg 298] to the face, of taste to the front of the tongue, and of control of muscles of mastication; afferent and efferent) connect with the skin of the face, the mucous membrane of the mouth, the teeth, and the muscles of mastication.
5. The seventh pair (facial nerves; control muscles that give the facial expressions; efferent) connect with the muscles just beneath the skin of the face.
6. The eighth pair (auditory nerves; nerves of hearing; afferent) connect with the internal ear.
7. The ninth pair (glossopharyngeal nerves; nerves of taste to back of tongue and of muscular control of pharynx; afferent and efferent) connect with the back surface of the tongue and with the muscles of the pharynx.
8. The tenth pair (vagus, or pneumogastric, nerves; nerves of feeling and of muscular control; afferent and efferent) connect with the heart, larynx, lungs, and stomach. They have the widest distribution of any of the cranial nerves.
9. The eleventh pair (spinal accessory nerves; control muscles of neck; efferent) connect with the muscles of the neck.
10. The twelfth pair (hypoglossal nerves; control muscles of the tongue; efferent) connect with the muscles of the tongue.
Sympathetic Ganglia and Nerves.—The sympathetic ganglia are found in different parts of the body, and vary in size from those which are half an inch in diameter to those that are smaller than the heads of pins. The largest and most important ones are found in two chains which lie in front, and a little to either side, of the spinal column, and extend from the neck to the region of the pelvis (Figs. 125 and 133). The number of ganglia in each of these chains is about twenty-four. They are connected[pg 299] on either side by the right and left sympathetic nerves which extend vertically from ganglion to ganglion. In addition to the ganglia forming these chains, important ones are found in the head (outside of the cranial cavity) and in the plexuses of the thorax and the abdomen.
The sympathetic ganglia receive nerves from the central division of the nervous system, but connect with glands, blood vessels, and the intestinal walls through fibers from their own cell-bodies. Some of these latter fibers join the spinal nerves, and some blend with each other to form small sympathetic nerves.
Protection of Brain and Spinal Cord.—On account of their delicate structure, the brain and spinal cord require the most complete protection. In the first place, they are surrounded by the bones of the head and spinal column; these not only shield them from the direct effects of physical force, but by their peculiar construction prevent, to a large degree, the passage of jars and shocks to the parts within. In the second place, they are surrounded by three separate membranes, as follows:
1. The dura, or dura mater, a thick, dense, and tough membrane which lines the bony cavities and forms supporting partitions.
2. The pia, or pia mater, a thin, delicate membrane, containing numerous blood vessels, that covers the surface of the brain and cord.
3. The arachnoid, a membrane of loose texture, that lies between the dura and the pin.
Finally, within the spaces of the arachnoid is a lymph-like liquid which completely envelops the brain and the cord, and which, by serving as a watery cushion, protects them from jars and shocks. Thus the brain and cord are directly shielded by bones, by membranes, and by the [pg 300]liquid which surrounds them. They are also protected from jars resulting from the movements of the body by the general elasticity of the skeleton.
Summary.—The nervous system establishes connections between all parts of the body, and provides a stimulus by means of which they are controlled. It is made up of a special form of cells, called neurons. The neurons form the different divisions of the nervous system, and also serve as the active agents in carrying on its work. Through a side-by-side method of joining they form the nerves, ganglia, spinal cord, and brain; and by a method of end-to-end joining they connect places remote from each other, and provide for nervous movements through the body. The nervous system, may in some respects be compared to a complicated system of telephony, in which the chains of neurons correspond to the wires, and the brain and spinal cord to the central station.
Exercises.—1. Give the meaning of the term "coördination." Supply illustrations.
2. What two general conditions are supplied in the body by the nervous system?
3. Compare the skeleton outline of the nervous system with the bony skeleton.
4. Sketch outlines of mon-axonic and di-axonic neurons.
5. Give two differences between the neurons and the other cells of the body.
6. Describe the two general methods of connecting neurons in the body. What purpose is accomplished by each method?
7. Name and locate the principal divisions of the nervous system.
8. Draw an outline of the brain (side view), locating each of its principal divisions.
9. If a pencil were placed over the ear, what portions of the brain would be above it and what below?
10. Describe briefly the cerebrum, the cerebellum, the midbrain, the pons, and the bulb.
[pg 301]11. Locate and describe the cortex. State purpose of the convolutions.
12. State the general differences between the cranial and the spinal nerves.
13. Locate and give the number of the dorsal-root ganglia. Locate and give the approximate number of the sympathetic ganglia.
14. Show how the two portions of the spinal nerves are formed—the one from the mon-axonic and the other from the di-axonic neurons.
15. Enumerate the different agencies through which the brain and spinal cord are protected.
16. What cranial nerves contain afferent fibers? What ones contain efferent fibers? What ones contain both afferent and efferent fibers?
17. In what respects is the nervous system similar to a system of telephony? In what respects is it different?
PRACTICAL WORK
Examine a model of the brain, identifying the different divisions and noting the position and relative size of the different parts (Fig. 137). Observe the convolutions of the cerebrum and compare these with the parallel ridges of the cerebellum. If the model is dissectible, study the arrangement of the cell-bodies (gray matter) and the distribution of the fiber bundles (white matter). Note the connection of the cranial nerves with the under side.
Fig. 137—Model for demonstrating the brain (dissectible).
A prepared nervous system of a frog (such as may be obtained from supply houses) should also be examined. Observe the appearance and general distribution of the nerves and their connection with the brain and spinal cord. If such a preparation is not at hand, some small animal may be dissected to show the main divisions of the nervous system, as follows:
Dissection of the Nervous System (by the teacher).—For this purpose a half-grown cat is generally the best available material. This [pg 302]should be killed with chloroform and secured to a board as in the dissection of the abdomen (page 169). Open the abdominal cavity and remove the contents, tying the alimentary canal where it is cut, and washing out any blood which may escape. Dissect for the nervous system in the following order:
1. Cut away the front of the chest, exposing the heart and lungs. Find on each side of the heart a nerve which passes by the side of the pericardium to the diaphragm. These nerves assist in controlling respiration and are called the phrenic nerves. Find other nerves going to different parts of the thorax.
2. Remove the heart and lungs. Find in the back part of the thoracic cavity, on each side of the spinal column, a number of small "knots" of nervous matter joined together by a single nerve. These are sympathetic ganglia. Where the neck joins the thorax, find two sympathetic ganglia much larger than the others.
3. Cut away the skin from the shoulder and upper side of the fore leg. By separating the muscles and connective tissue where the leg joins the thorax, find several nerves of considerable size. These connect with each other, forming a network called the brachial plexus. From here nerves pass to the thorax and to the fore leg.
4. From the brachial plexus trace out the nerves which pass to different parts of the fore leg. In doing this separate the muscles with the fingers and use the knife only where it is necessary to expose the nerves. Note that some of the branches pass into the muscles, while others connect with the skin.
5. Remove the skin from the upper portion of one of the hind legs and separate the muscles carefully until a large nerve is found. This is one of the divisions of the sciatic nerve. Carefully trace it to the spinal cord, cutting away the bone where necessary, and find the connections of its branches with the cord. Then trace it toward the foot, discovering its branches to different muscles and to the skin.
6. Unjoint the neck and remove the head. Examine the spinal cord where exposed. Cut away the bone sufficiently to show the connection between the cord and one of the spinal nerves. On the dorsal root of one of the nerves find a small ganglion. What is it called?
7. Fasten the head to a small board and remove the scalp. Saw through the skull bones in several directions. Pry off the small pieces of bones, exposing the upper surface of the brain. Study its membranes, convolutions, and divisions.
[pg 303]8. With a pair of bone forceps, or nippers, break away the skull until the entire brain can be removed from the cavity. Examine the different divisions, noting the relative position and size of the parts.
9. With a sharp knife cut sections through the different parts, showing the positions of the "gray matter" and of the "white matter."
NOTE.—If the entire class is to examine one specimen, it is generally better to have the dissecting done beforehand and the parts separated and tacked to small boards. This will permit of individual examination. Sketches of the sciatic nerve, brachial plexus, and of sections through the brain and spinal cord should be made.
Location of Nerves in the Body.—Several of the nerves of the body lie sufficiently near the surface to be located by pressure and are easily recognized as sensitive cords. Slight pressure from the fingers reveals the presence of nerves in the grooves of the elbow (the crazy bone), between the muscles on the inner side of the arm near the shoulder, and in the hollow part of the leg back of the knee. These are all large nerves. Small nerves may be located in the same manner in the face and neck.