584 THE BRAIN. 



CHAPTER II. 



THE BRAIN. 

 ON SOME GENERAL FEATURES OF THE STRUCTURE OF THE BRAIN. 



513. IT would be out of place to attempt to. give here a complete de- 

 scription of the structure of the brain ; but certain features must be kept 

 fresh in the mind as a basis for physiological discussion ; and to these we 

 must now turn our attention, a general acquaintance with the topographical 

 anatomy of the brain being presupposed. 1 



Like the spinal cord, the brain consists of " white matter," in which the 

 nervous elements are almost exclusively medullated fibres, and of " gray 

 matter," in which nerve-cells and other nervous elements are also present ; 

 but the gray matter of the brain is much more variable in structure than 

 that of the spinal cord, and possesses features peculiar to itself; these we 

 shall study later on. 



For physiological purposes the brain may be conveniently divided into 

 parts corresponding to the divisions which appear in it in the embryo. At 

 an early stage in the life of the embryo, that part of the medullary tube 

 which is about to become the brain differs from that which is about to 

 become the spinal cord, in that the central canal, which in the latter is of 

 fairly uniform bore along its whole length, is in the former alternately 

 widened and narrowed, so that the tube forms a series of vesicles, the cerebral 

 vesicles, succeeding each other lengthways. At first these vesicles are three 

 in number, called respectively fore-brain, mid-brain, and hind-brain ; but the 

 fore-brain, after having developed on each side a lateral vesicle, the optic 

 vesicle, subsequently transformed into the retina and optic nerve, gives rise 

 in front of itself to a pair of vesicles placed side by side, or rather to a single 

 vesicle with a deep median furrow, the vesicle of the cerebrum, containing a 

 cavity divided by a median partition into two cavities, lying side by side, 

 which open into the cavity of the original fore-brain by a Y-shaped opening. 

 This embryonic chain of vesicles is developed into the adult brain by unequal 

 growth of the walls and unequal expansion of the cavities, certain features 

 being also impressed upon it by the bend on the longitudinal axis, which 

 takes place in the region of the mid-brain and is known as the cranial 

 flexure. 



514. In the hind part of the hinder vesicle or hind-brain, the ventral, 

 basal portion or floor is thickened to form the bulb, while the greater part of 

 the dorsal portion or roof does not thicken at all, is not transformed into 

 nervous elements, but remains as a single layer of epithelium, adherent to 

 the pia mater overlying it, and so forms a thin covering to the lozenge- 

 shaped cavity of the vesicle, now known as the fourth ventricle. 



In the front part of the same hind-brain, on the contrary, the roof and 

 sides are enormously developed into the conspicuous cerebellum overhanging 

 the front part of the fourth ventricle, while the floor is also thickened into 

 the pons Varolii. 



This thickening of the pons is largely made up, on the one hand, of hori- 

 zontal nerve-fibres, which run transversely from each side of the cerebellum 

 into the pons, or from one side of the cerebellum to the other, and, on the 

 other hand, of longitudinal fibres, which run forward from the bulb and are 



1 Figs. 131 and 140. which will be found in succeeding sections, may with advantage 

 be consulted in reading this section, though not specially referred to in the text. 



