CHAPTER II. 

 THE BRAIN. 



SEC. 1. ON SOME GENERAL FEATURES OF THE 

 STRUCTURE OF THE BRAIN. 



600. It would be out of place to attempt to give here a 

 complete description 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 'grey matter,' in which nerve-cells and other nervous 

 elements are also present ; but the grey 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 di- 

 vided 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 



1 Figs. 108 to 123 which will be found in succeeding sections may with 

 advantage be consulted in reading this section though not specially referred to in 

 the text. 



592 



