As we pass upward in attempting to trace the conducting channels of the medulla, we come to the more elaborate system of nervous textures which, together, are called the brain. This is anatomically the most highly developed, and physiologically the most intricate, part of the central nervous organs. Besides the nerve cells and various kinds of conducting channels with which we have already become familiar in the cord, etc., there are in the brain a vast number of smaller elements which do not possess the distinctive characteristics of nerve cells. These granular bodies are tightly packed together in many parts of the centres, and must have some important function.
To form a general idea of the plan of construction of the brain, it is well to follow its development in the earlier stages of the embryo, from the time when it forms an irregular and thickened part of the tube of tissue, from which is developed the cerebrospinal axis. From this it will be seen that the brain is but a modified part of the primitive nervous tube, in which swellings may be observed at an early period of embryonic life. These swellings are called the fore-brain, mid-brain and hind-brain, and in the future development of the parts give rise to (1) the hemispheres and basal ganglia; (2) the corpora quadrigemina, pons and cerebellum; and (3) the medulla oblongata. The great mass of the brain - the hemispheres - is formed by an excessive development of bud-like processes which grow out from the sides of the fore-brain at an early period, and become elaborately folded, so that in the adult it is difficult to trace the relationship to the original form. For further details of the development of the brain, vide chapter on that subject.
The cells of the brain are, like those of the cord, grouped together in the complex gray substance, while the white part is made up of conducting fibres. The gray substance is distributed in four distinct regions. (1) Of these one can be traced along the floor of the fourth ventricle, from the gray matter of the cord to the base of the brain, as far forward as the tuber cine-reum, so that it may be considered representative of the gray matter forming the inner lining of the primitive nervous tube. (2) The ganglia of the brain are isolated masses of gray substance within the brain, known as the corpora quadrigemina, optic thalami, corpora striata, etc. (3) The gray substance of the cerebellum and of the corpora quadrigemina is derived from the upper part of the mid-brain. (4) The cortex of the hemispheres of the brain is the most extensive gray district, and must be regarded as distinct from the preceding.
Fig.253. Diagram illustrating the progressive changes in the development of the brain.
1. Shows the first step; the formation of three vesicles.
2. Shows the budding forward of the hemispheres (cr), upward of the pineal gland (p/),and downward of the pituitary body (pt) from the fore-brain (a), and the thickening of the mid-(b) and hind-brain (c).
3. Shows the backward turn of the hemispheres and their cavity (lateral ventricle) (/). The development of the corpora quadrigemina (q) and crura (r) from the mid-brain (b) and the cerebellum (cd) and pons (p) from the hind-brain; their cavities being narrowed into the iter a tertio ad quartum ventricultim.
4. The hemispheres now extend backward and form the temporal lobe (tl), which afterward grows downward and forward. The fornix (f) approaches its final position. The space (x) between the fornix and velum is closing, so that the outside of the brain (morphologically) becomes practically its very centre.
a, Fore-brain; b, Mid-brain; c, Hind-brain; cb, Cerebellum; cr, Cerebrum; d, Cavity of medulla; f, Fornix; /, Lateral ventricle; m, Medulla oblongata; ma, Corpora mammillaria; o, Olfactory lobe; p, Pons Varolii; pl, Pineal gland; pt, Pituitary body; q, Corpora quadrigemina; r, Crura cerebri; t, Lamina terminalis; tl, Temporal lobe 0f cerebrum; x, Space enclosed by the extension backward of the cerebrum.
Fig. 254. Diagram of some of the paths taken by nerve impulses in the brain and spinal cord.
c. Gray substance of cerebral cortex, c'. Gray substance of cerebellum.
cr. Cranial nerves, some afferent and some efferent.
M. Motor (efferent) spinal nerves. S. Sensory (afferent) spinal nerves.
Connecting the various parts of these gray regions are sets of fibres, which may be classified as follows: -
1. Those which act as channels of intercommunication for the different parts of the same region. These may be divided into unilateral, which connect together the cells of a single hemisphere, and bilateral, or commissural fibres, which unite the corresponding masses of gray matter on both sides of the brain.
2. Those which connect the different regions one with another. Under this head fall (1) those fibres which pass between the cortex and the basal ganglia, or anterior gray column of the spinal cord; (2) those running from the cortex or the spinal cord to the cerebellum; and (3) those connecting the above with the spinal gray matter.