The brain, like the rest of the nervous system, is developed from the ectoderm or outer layer of the embryo by the formation of a groove in the mid-dorsal line. The lips of this medullary groove unite to form a canal beginning at the place where the neck of the embryo is to be. The part of the neural canal in front of the earliest union forms the brain and very early becomes constricted into three vesicles, to which the names of prosencephalon, mesencephalon and rhombencephalon are now usually given. The simple tubular brain we have seen as a permanent arrangement in Amphioxus, but the stage of the three vesicles is a transitory one, and is not found in the adult of any existing animal. From the sides of the prosencephalon, the optic vesicles grow out before the neural tube is completely closed, and eventually form the optic nerves and retinae, while, soon after this, the cerebral hemispheres bulge from the antero-dorsal part of the first primary vesicle, their points of evagination being the foramina of Munro. From the ventral parts of these cerebral hemispheres the olfactory lobes are constricted off, while just behind the openings of the foramina of Munro a constriction occurs which divides the prosencephalon into two secondary vesicles, the anterior of which, containing the foramina of Munro, is called the telencephalon, while the posterior is the thalamencephalon or diencephalon. A constriction also occurs in the hind vesicle or rhombencephalon, dividing it into an anterior part, the metencephalon, from which the cerebellum is developed, and a posterior or myelencephalon, the primitive medulla oblongata. At this stage the general resemblance of the brain to that of the lamprey is striking.
Before the secondary constrictions occur three vertical flexures begin to form. The first is known as the cephalic, and is caused by the prosencephalon bending sharply downward, below and in front of the mesencephalon. The second is the cervical, and marks the place where the brain ends and the spinal cord begins; the concavity of this flexure is ventral. The third to appear has a ventral convexity and is known as the pontine, since it marks the site of the future pons Varolii; it resembles the permanent flexure in the reptilian brain.
| From Cat. R.C.S. England. Fig. 23.—Lateral view of cerebral hemisphere of Gorilla (Anthropopithecus gorilla). |
It will now be seen that the original neural canal, which is lined by ciliated epithelium, forms the ventricles of the brain, while superficial to this epithelium (ependyma) the grey and white matter is subsequently formed. It has been shown by His that the whole neural tube may be divided into dorsal or alar, and ventral or basal laminae, and, as the cerebral hemispheres bud out from the dorsal part of the anterior primary vesicle, they consist entirely of alar laminae. The most characteristic feature of the human and anthropoid brain is the rapid and great expansion of these hemispheres, especially in a backward direction, so that the mesencephalon and metencephalon are hidden by them from above at the seventh month of intra-uterine life. At first the foramina of Munro form a communication not only between the third and lateral ventricles, but between the two lateral ventricles, so that the cavity of each hemisphere is continuous with that of the other; soon, however, a median longitudinal fissure forms, into which the mesoderm grows to form the falx, and so the foramina of Munro are constricted into a V-shaped canal. In the floor of the hemispheres the corpora striata are developed at an early date by a multiplication of nerve cells, and on the external surface a depression, called the Sylvian fossa, marks the position of the future central lobe, which is afterwards hidden as the lips of the fossa (opercula) gradually close in on it to form the Sylvian fissure. The real fissures are complete infoldings of the whole thickness of the vesicular wall and produce swellings in the cavity. Some of them, like the choroidal on the mesial surface, are developed very early, while the vesicle is little more than epithelial, and contain between their walls an inpushing of mesoderm to form the choroid plexus. Others, like the hippocampal and calcarine, appear in the second and third months and correspond to invaginations of the nervous tissue, the hippocampus major and minor. The sulci appear later than the fissures and do not affect the internal cavity; they are due to the rapid growth of the cortex in certain areas. The corpus callosum and fornix appear about the third month and their development is somewhat doubtful; they are probably modifications of the lamina terminalis, but they may be secondary adhesions between the adjacent surfaces of the cerebral hemispheres where the cortical grey matter has not covered the white. They begin at their antero-ventral part near the genu of the corpus callosum and the anterior pillars of the fornix, and these are the parts which first appear in the lower mammals. The original anterior vesicle from which the hemispheres evaginate is composed, as already shown, of an anterior part or telencephalon and a posterior or thalamencephalon; the whole forming the third ventricle in the adult. Here the alar and basal laminae are both found, but the former is the more important; from it the optic thalami are derived, and more posteriorly the geniculate bodies. The anterior wall, of course, is the lamina terminalis, and from it are formed the lamina cinerea, the corpus callosum, fornix and septum lucidum. The roof largely remains epithelial and is invaginated into the ventricle by the mesoderm to form the choroid plexuses of the third ventricle, but at the posterior part it develops the ganglia habenulae and the pineal body, from a structure just in front of which both a lens and retinal elements are derived in the lower forms. This is one great difference between the development of this organ and that of the true eyes; indeed it has been suggested that the pineal is an organ of thermal sense and not the remains of a median eye at all. The floor of the third ventricle is developed from the basal laminae, which here are not very important and from which the tuber cinereum and, until the fourth month, single corpus mammillare are developed. The infundibulum or stalk of the posterior part of the pituitary body at first grows down in front of the tuber cinereum and, according to Gaskel’s theory, represents an ancestral mouth to which the ventricles of the brain and the central canal of the cord acted as the stomach and intestine (Quart. Journ. of Mic. Sci. 31, p. 379; and Journ. of Phys. v. 10, p. 153). The reason why the basal lamina is here small is because it contains the nuclei of no cranial nerves. The anterior and posterior commissures appear before the middle and the middle before the corpus callosum, as they do in phylogeny. In connexion with the thalamencephalon, though not really belonging to it, may be mentioned the anterior lobes of the pituitary body; these begin as an upward diverticulum from the posterior wall of the primitive pharynx or stomatodaeum about the fourth week. This pouch of Rathke, as it is called, becomes nipped off by the developing base of the skull, and its bifid blind end meets and becomes applied to the posterior part of the body, which comes down from the brain. In the mesencephalon the alar laminae form the corpora quadrigemina; these at first are bigeminal and hollow as they are in the lower vertebrates. The basal laminae thicken to form the crura cerebri. In the rhombencephalon the division into basal and alar laminae is better marked than in any other part; there is a definite groove inside the fourth ventricle, which remains in the adult as the superior and inferior fovea and which marks the separation between the two laminae. In the basal laminae are found the deep origins of most of the motor cranial nerves, while those of the sensory are situated in the alar laminae. The roof of the fourth ventricle widens out very much and remains largely epithelial as the superior and inferior medullary vela. The cerebellum develops in the anterior part of the roof of the rhombencephalon as two lateral rudiments which unite in the mid line and so form a transverse bar similar to that seen in the adult lamprey; at the end of the second month the flocculus and paraflocculus become marked, and later on a series of transverse fissures occur dividing the various lobes. Of the cerebellar peduncles the inferior develops first (third month), then the middle forming the pons (fourth month), and lastly the superior (fifth month) (Elliot Smith, Review of Neurology and Psychiatry, October 1903; W. Kuithan, “Die Entwicklung des Kleinhirns bei Säugetieren,” Munchener Med. Abhandl., 1895; B. Stroud, “Mammalian cerebellum,” Journ. of Comp. Neurology, 1895). Much of our knowledge of the tracts of fibres in the brain is due to the fact that they acquire their white sheaths at different stages of development, some long after birth.
For further details and references see Quain’s Anat. vol. i. (1908); Minot’s Human Embryology (New York); W. His, Anat. menschlicher Embryonen (Leipzig, 1881); Marshall’s Vertebrate Embryology; Kölliker, Grundriss der Entwickelungsgeschichte (Leipzig, 1880); A. Keith, Human Embryology and Morphology (London, 1904); O. Hertwig, Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbeltiere, Bd. 2, part 3 (Jena, 1902-1906); Development of the Human Body, J.P. McMurrich (1906).
(F. G. P.)
2. Physiology
The nervous system has as its function the co-ordinating of the activities of the organs one with another. It puts the organs into such mutual relation that the animal reacts as a whole with speed, accuracy and self-advantage, in response to the environmental agencies which stimulate it. For this office of the nervous system there are two fundamental conditions. The system must be thrown into action by agencies at work in the environment. Light, gravity, mechanical impacts, and so on, which are conditions significant for animal existence, must find the system responsive and through it evoke appropriate activity in the animal organs. And in fact there have been evolved in the animal a number of structures called receptive organs which are selectively excitable by different environmental agencies. Connected with these receptive organs lies that division of the nervous system which is termed afferent because it conducts impulses inwards towards the nervous centres. This division consists of elongated nerve-cells, in man some two million in number for each half of the body. These are living threads of microscopic tenuity, each extending from a receptive organ to a central nervous mass. These central nervous masses are in vertebrates all fused into one, of which the part which lies in the head is especially large and complex, because directly connected with particularly important and delicate receptive organs. The part of the central nervous organ which lies in the head has, in consequence of its connexion with the most important receptive organs, evolved a dominant importance in the nervous system, and this is especially true of the higher animal forms. This head part of the central nervous organ is sufficiently different from the rest, even to anatomical examination, to have received a separate name, the brain. But the fact of its having received a separate name ought not to obscure the singleness and solidarity of the whole central nervous organ as one entity. The functions of the whole central nervous organ from region to region are essentially similar throughout. One of its essential functions is reception, via afferent nerves, of nervous impulses generated in the receptive organs by environmental agents as stimuli. In other words, whatever the nature of the agent, its result on the receptive organs enters the central nervous organ as a nervous impulse, and all segments of the central nervous organ receive impulses so generated. Further, it is not known that nervous impulses present qualitative differences among themselves. It is with these impulses that the central nervous organ whether spinal cord or brain has to deal.
Material and Psychical Signs of Cerebral Activity.—In the central nervous organ the action resulting from entrant impulses has issue in three kinds of ways. The reaction may die out, be suppressed, and so far as discoverable lead to nothing; or the impulses may evoke effect in either or both of two forms. Just as from the receptive organs, nerves lead into the central nervous organ, so conversely from the central organ other nerves, termed efferent, lead to various organs of the body, especially glands and muscles. The reaction of the central nervous organ to impulses poured into it commonly leads to a discharge of impulses from it into glands and muscles. These centrifugal impulses are, so far as is known, qualitatively like the centripetal impulses. On reaching the glands and muscles they influence the activity of those organs. Since those organs are therefore the mechanisms in which the ultimate effect of the nervous reaction takes place, they are often termed from this point of view effector organs. A change ensuing in effector organs is often the only sign an observer has that a nervous reaction has occurred, unless the nervous system under observation be the observer’s own.
If the observer turns to his own nervous system for evidence of reaction, he meets at once in numberless instances with sensation as an outcome or sign of its reaction. This effect he cannot show to any being beside himself. He can only describe it, and in describing it he cannot strictly translate it into any term of material existence. The unbridged gulf between sensation and the changes produced in effector organs necessitates a separate handling of the functions of the nervous system according as their office under consideration is sensation or material effect. This holds especially in the case of the brain, and for the following reasons.