CHAPTER XXXVI.
THE NERVOUS SYSTEM.
725. In the preceding chapters, we have seen how various and complex are many of the motions necessary to maintain the life of an animal whose organization is superior to all others. We have noted the wonderful mechanism of the muscular system, in producing the varied movements of the body, the different processes by which the food is converted into chyle and mixed with the blood, and the circulation of this fluid to every organ and tissue of the system, that each may select from it the very principles which it requires for its growth.
726. Lymphatic absorption commences as soon as nutrition is completed, and conveys the useless, worn-out particles of different tissues back into the circulating fluid; while the respiratory organs and secretory glands perform the work of preparing the waste products to be eliminated from the body. Each of these processes effects a single object, and is performed in a regular manner.
727. “They must succeed each other in proper order in propelling every particle to its proper destination, or life would be sacrificed almost at the moment of its commencement. There is, therefore, a mutual dependence of all portions of the machinery of organic life upon each other, and a necessity for some medium of communication from one organ to another, by which they may convey mutual information of 328 their several conditions, if we may be permitted to employ a figurative expression. Were there no such medium, how would the stomach notify the heart that additional exertion on its part is required, because the stomach is busy in digesting food?
725. What has been noted in the preceding chapters? 726. Show the manner in which the several processes are performed. 727. How must they succeed each other?
728. “When we are exerting the muscular system for a long time in some laborious employment, how else are our members to inform the stomach that they are too much occupied with their duties to spare the blood necessary in digestion; that it is requisite that the appetite should decline; and that digestion should cease for the time, even if the stomach should be oppressed with its contents? When we are thinking, how else are the blood-vessels to be told that an unusual supply of their contents is wanting in the head? or when the whole frame is weary with exertion, how, without some regular line of intelligence between all the organs, is the brain to be instructed that circumstances require that it should go to sleep? To supply the necessary medium of communication, Providence has furnished all the animals that possess distinct organs, with a peculiar apparatus called the Nervous System.”
ANATOMY OF THE NERVOUS SYSTEM.
729. The NERVOUS SYSTEM consists of the Cer´e-bro-spi´nal Cen´tre, and of numerous rounded and flattened white cords, called nerves, which are connected at one extremity with the cerebro-spinal centre, and at the other, distributed to all the textures of the body. The sympathetic nerve is an exception to this description; for, instead of one, it has many small centres, which are called gan´gli-a, and which communicate very freely with the cerebro-spinal centre, and with its nerves.
728. What is the medium of communication from one organ to another? 729–754. Give the anatomy of the brain and cranial nerves. 729. Of what does the nervous system consist? What constitutes an exception to this?
730. The CEREBRO-SPINAL CENTRE consists of two portions: The brain, and the spinal cord. For convenience of description, the nervous system may be divided into the Brain, Cranial Nerves, Spinal Cord, Spinal Nerves, and the Sympathetic Nerve.
731. The term BRAIN designates those parts of the nervous system, exclusive of the nerves themselves, which are contained within the cranium, or skull-bones; they are the Cer´e-brum, Cer-e-bel´lum, and Me-dul´la Ob-lon-ga´ta. These are invested and protected by the membranes of the brain, which are called the Du´ra Ma´ter, A-rach´noid, and Pi´a Ma´ter.
Fig. 120.
Fig. 120. 1, 1, The scalp turned down. 2, 2, 2, The cut edge of the bones of the skull. 3, The external strong membrane of the brain (dura mater,) suspended by a hook. 4, The left hemisphere of the brain, showing its convolutions.
730. Of what does the cerebro-spinal centre consist? How is the nervous system divided? 731. What does the term brain designate? Name them. How are they protected? Describe fig. 120.
732. The CEREBRUM IS divided into two hemispheres, by a cleft, or fissure. Into this cleft dips a portion of the dura mater, called the falx cer´e-bri, from its resembling a sickle. The apparent design of this membrane is to relieve the one side from the pressure of the other, when the head is reclining to either side. Upon the superior surface of the cerebrum are seen undulating windings, called con-vo-lu´tions. Upon its inferior, or lower surface, each hemisphere admits of a division into three lobes—the anterior, middle, and posterior. (Fig. 122, 123)
Fig. 121
Fig. 121. A section of the skull-bones and cerebrum. 1, 1, The skull. 2, 2, the dura mater. 3, 3, The cineritious portion of the cerebrum. 4, 4, The medullary portion. The dark points indicate the position of divided blood-vessels. 5, 5, The lateral ventricles.
732. How is the cerebrum divided? What is the use of the falx cerebri? What is seen upon the superior surface of the brain? Its inferior?
733. When the upper part of the hemispheres is removed horizontally with a sharp knife, a centre of white substance is brought to view. This is surrounded by a border of gray, which follows the depressions of the convolutions, and presents a zigzag outline. The divided surface will be seen studded with numerous small red points, which are produced by the escape of blood from the division of the minute arteries and veins. The gray border is called the cortical, or cineritious portion, while the white central portion is called the medullary. The two hemispheres are connected by a dense layer of transverse fibres, called cor´pus cal-lo´sum.
734. In the interior of the brain there are several cavities, two of which are of considerable size, and are called the lateral ventricles. They extend from the anterior to the posterior part of the brain, and wind their way into other parts of the cerebral organ.
Observation. In the disease called “dropsy of the brain,” (hydrocephalus internus,) the serum, or water, is usually deposited in these ventricles. This is effused from the many small blood-vessels of the membrane in these cavities.
735. The brain is of a pulpy character, quite soft in infancy and childhood; but it gradually becomes more and more consistent, and in middle age it assumes the form of determinate structure and arrangement. It is more abundantly supplied with blood than any organ of the system. No lymphatics have been detected, but it is to be presumed that they exist in this organ.
736. The CEREBELLUM is about seven times smaller than the cerebrum. Like that organ, it is composed of white and 332 gray matter, but the gray constitutes the larger portion. Its surface is formed of parallel plates separated by fissures. The white matter is so arranged, that when cut vertically, the appearance of the trunk and branches of a tree (ar´bor vi´tæ) is presented. It is situated under the posterior lobe of the cerebrum, from which it is separated by a process of the dura mater, called the ten-to´ri-um.
733. Describe the appearance of the brain when a horizontal section has been made. What is the gray border often called? What connects the hemispheres? 734. Describe the ventricles of the brain. In the disease called “dropsy of the brain,” where is the water deposited? 735. What is the character of the brain in childhood? In adults? 736. How does the cerebellum compare in size with the cerebrum?
Fig. 122.
Fig. 122. The under surface, or base, of the brain and origin of the cranial nerves. 1, 1, The anterior lobes of the cerebrum. 2, 2, The middle lobes. 3, 3, The posterior lobes, almost concealed by the cerebellum. 4, 4, The cerebellum. 7, 7, The longitudinal fissure that divides the brain into two hemispheres. 8, The first pair of nerves. 9, 9, The second pair of nerves. 10, The decussation, or crossing, of its fibres. 13, 13, The third pair of nerves. 14, The pons varolii. 15, 15, The fourth pair of nerves. 16, 16, The fifth pair of nerves. 17, The sixth pair of nerves. 18, 18, The seventh and eighth pair of nerves. 19, The medulla oblongata, with the crossing of some of its fibres exhibited. 20, The ninth pair of nerves. 21, The tenth pair of nerves, 22, The eleventh pair of nerves. 23, The twelfth pair of nerves.
Describe this portion of the brain. Explain fig. 122.
737. The MEDULLA OBLONGATA, or that portion of the spinal cord which is within the skull, consists of three pairs of bodies, (cor´pus py-ram-i-da´le, res-ti-for´me, and ol-i-va´re,) united in a single bulb.
Fig. 123.
Fig. 123. The base of the skull and the openings through which the cranial nerves pass. 1, 1, The first pair of nerves. 2, 2, The cribriform plate of the ethmoid bone through which this nerve passes. 3, 3, The second pair of nerves. 4, 4, The optic foramen in the sphenoid bone; through which passes the second pair of nerves. 5, 5, The sphenoidal fissure. 6, 6, The third pair of nerves. 7, 7, The fifth pair of nerves. 8, 8, The ophthalmic branch of the fifth nerve. The third, the ophthalmic branch of the fifth and the sixth nerve pass from the brain through the sphenoidal fissure to the eye. 9, 9, The superior maxillary branch of the fifth nerve. 10, 10, The foramen rotundum, (round opening,) through which the nerve 9, 9, passes to the upper jaw. 11, 11, The inferior maxillary branch of the fifth pair. 12, 12, The foramen ovale, (oval opening,) through which the nerve 11, 11, passes to the lower jaw. 13, 13, The sixth pair of nerves. 14, 14, The seventh and eighth pair of nerves. 15, 15, The opening in the temporal bone, through which the seventh and eighth nerves pass to the face and ear. 16, 16, The ninth pair of nerves. 17, The tenth pair of nerves. 18, 18, The eleventh pair of nerves. 19, 19, The foramen lacerum (rough opening.) The ninth, tenth, and eleventh nerves pass from the brain through this opening. 20, The spinal cord. 21, The foramen spinalis, through which the spinal cord passes. 22, 22, The position of the anterior lobe of the brain. 23, 23, The middle lobe. 24, 24, The posterior lobe. 25, 25, A section of the skull-bones.
737. Describe the medulla oblongata. Explain fig. 123.
738. The DURA MATER is a firm, fibrous membrane, which is exposed on the removal of a section of the skull-bones. This lines the interior of the skull and spinal column, and likewise sends processes inward, for the support and protection of the different parts of the brain. It also sends processes externally, which form the sheaths for the nerves, as they quit the skull and spinal column. The dura mater is supplied with arteries and nerves.
Fig. 124.
Fig. 124. A vertical section of the cerebrum, cerebellum, and medulla oblongata, showing the relation of the cranial nerves at their origin. 1, The cerebrum. 2, The cerebellum, with its arbor vitæ represented. 3, The medulla oblongata. 4, The spinal cord. 5, The corpus callosum. 6, The first pair of nerves. 7, The second pair. 8, The eye. 9, The third pair of nerves. 10, The fourth pair. 11, The fifth pair. 12, The sixth pair. 13, The seventh pair. 14, The eighth pair. 15, The ninth pair. 16, The tenth pair. 10, The eleventh pair. 18, The twelfth pair. 20, Spinal nerves. 21, The tentorium.
738. Describe the dura mater. What is its use? Explain fig. 124.
739. The ARACHNOID, so called from its extreme tenuity, is the serous membrane of the brain and spinal cord, and is, like other serous membranes, a closed sac. It envelops these organs, and is reflected upon the inner surface of the dura mater, giving to that membrane its serous investment.
740. The PIA MATER is a vascular membrane, composed of innumerable vessels, held together by cellular membrane. It invests the whole surface of the brain, and dips into its convolutions. The pia mater is the nutrient membrane of the brain, and receives its blood from the carotid and vertebral arteries. Its nerves are minute branches of the sympathetic, which accompany the branches of the arteries.
741. The CRANIAL NERVES, that connect with the brain, are arranged in twelve pairs. They are called: 1st. The Ol-fact´o-ry. 2d. The Op´tic. 3d. The Mo-to´res Oc-u-lo´rum. 4th. The Pa-thet´i-cus. 5th. The Tri-fa´cial. 6th. The Ab-du-cen´tes. 7th. The Por´ti-o Du´ra. 8th. The Por´ti-o Mol´lis. 9th. The Glos´so-pha-ryn´gi-al. 10th. The Pneu-mo-gas´tric. 11th. The Spi´nal Ac´ces-so-ry. 12th. The Hy´po-glos´sal.
742. The OLFACTORY NERVE (first pair) passes from the cavity of the skull through many small openings in a plate of the eth´moid bone. (This plate is called crib´ri-form, from its resemblance to a sieve.) This nerve ramifies upon the membrane that lines the nasal passages. It is the softest nerve of the body. (Fig. 136.)
743. The OPTIC NERVE (second pair) passes from the interior of the cranium, through an opening in the base of the skull, (fo-ra´men op´ti-cum,) to the cavity for the eye. It pierces the coats of the eye, and expands in the retina.
744. The MOTORES OCULORUM (third pair) pass from the 336 brain, through an opening of the sphe´noid bone, (sphe-noid´al fis´sure,) to the muscles of the eye.
739. Describe the arachnoid membrane. 740. What is said respecting the pia mater? 741. How many pairs of cranial nerves? Name them. 742. Describe the olfactory nerve. 743. The optic nerve. 744. Describe the motores oculorum.
745. The PATHETICUS (fourth pair) passes from the brain, through the sphenoidal fissure, to the superior oblique muscle of the eye.
Fig. 125.
Fig. 125. The distribution of the third, fourth, and sixth pairs of nerves, to the muscles of the eye. 1, The ball of the eye and rectus externus muscle. 2, The upper jaw. 3, The third pair, distributed to all the muscles of the eye, except the superior oblique, and external rectus. 4, The fourth pair passes to the superior oblique muscle. 6, The sixth pair, is distributed to the external rectus muscle.
746. The TRIFACIAL NERVE (fifth pair) is analogous to the spinal nerves in its origin by two roots, from the anterior and posterior columns of the spinal cord. It has a ganglion, like the spinal nerves upon its posterior root. For these reasons, it ranges with the spinal nerves, and is considered the cranial spinal nerve. This nerve divides into three branches:—The oph-thal´mic, superior max´il-la-ry, and inferior max´il-la-ry.
745. The patheticus. What does fig. 125 represent? 746. What is the trifacial nerve sometimes called? Why is it classed with the cranial spinal nerves? Give the names of its branches.
747. The ophthalmic nerve passes from the cranial cavity through the sphenoidal fissure. It sends branches to the forehead, eye, and nose. The superior maxillary nerve passes through an opening in the base of the skull, (foramen ro-tund´dum,) and sends branches to the eye, the teeth of the upper jaw, and the muscles of the face. The inferior maxillary nerve escapes from the cranial cavity through an opening called foramen o-va´le. It sends branches to the muscles of the lower jaw, the ear, the tongue, and the teeth of the lower jaw.
Fig. 126.
Fig. 126. The distribution of the fifth pair of nerves. 1, The orbit for the eye. 2, The upper jaw. 3, The tongue. 4, The lower jaw. 5, The fifth pair of nerves. 6, The first branch of this nerve, that passes to the eye. 9, 10, 11, 12, 13, 14, Divisions of this branch. 7, The second branch of the fifth pair of nerves is distributed to the teeth of the upper jaw. 15, 16, 17, 18, 19, 20, Divisions of this branch. 8, The third branch of the fifth pair, that passes to the tongue and teeth of the lower jaw. 23. The division of this branch that passes to the tongue, called the gust´a-to-ry. 24. The division that is distributed to the teeth of the lower jaw.
747. Where do the filaments of the ophthalmic branch ramify? The superior maxillary? The inferior maxillary? Explain fig. 126.
748. The ABDUCENTES (sixth pair) passes through the opening by which the carotid artery enters the cranial cavity. It is the smallest of the cerebral nerves, and is appropriated to the external straight muscle of the eye.
749. The PORTIO MOLLIS (seventh pair) enters the hard portion of the tem´po-ral bone at the internal auditory opening, and is distributed upon the internal ear. (Fig. 147, 148.)
Fig. 127.
Fig. 127. A representation of the distribution of the eighth pair of nerves with some branches of the fifth. 1, 2, 3, 5, 7, 9, Are branches of the eighth pair. They are distributed over the face in a radiated manner, which constitutes the pes anserinus, (foot of a goose.) The nerves 4, 6, 8, are branches of the fifth pair. 10, 11, 12, 13, 14, 15, 16, Are branches of nerves from the upper part of the spinal cord, (cervical.)
750. The FACIAL NERVE (eighth pair) passes from the skull through an opening situated below the ear, (mas´toid foramen.) It is distributed over the face, supplying the muscles with nervous filaments.
748. What is said of the abducentes, or sixth pair of nerves? 749. Of the portio mollis? Explain fig. 127. 750. Of the facial nerve?
751. The GLOSSO-PHARYNGEAL NERVE (ninth pair) passes from the brain, through an opening with the jugular vein, (foramen lac´e-rum.) It is distributed to the mucous membrane of the tongue and throat, and also to the mucous glands of the mouth.
752. the PNEUMOGASTRIC NERVE (tenth pair) escapes from the brain through the foramen lacerum. It sends branches to the larynx, pharynx, œsophagus, lungs, spleen, pancreas, liver, stomach, and intestines. (Fig. 132.)
753. The SPINAL ACCESSORY NERVE (eleventh pair) has its origin in the respiratory tract of the spinal cord. It connects with the ninth and tenth pairs of nerves, and is distributed to the muscles about the neck.
754. The HYPO-GLOSSAL NERVE (twelfth pair) passes from the brain, through a small opening, (con´dy-loid foramen.) It ramifies upon the muscles of the tongue, and is its motor nerve.
Observation. The cranial nerves, with the exception of the olfactory, optic, and auditory, connect with each other by means of filaments. They also send connecting nervous filaments to the upper spinal nerves, (cervical,) and the sympathetic nerve.
751. Describe the glosso-pharyngeal nerve. 752. The pneumogastric nerve. 753. The spinal accessory nerve. 754. The hypo-glossal nerve.