IV. VE1UEBRATA. 



533 



The gray matter surrounds the central canal, but extends on 

 either side dorsally and ventrally into the white matter, so that 

 in section it resembles somewhat the letter H, with its dorsal 

 (fig. 76, HH) and ventral horns (VH). By means of these horns 

 and the dorsal and ventral nerve roots arising from them, the white 

 matter on either side is divided into three tracts, the dorsal (H) > 

 ventral (s), and lateral (S) columns of the cord. 



Corresponding to each muscle segment two nerve roots arise- 

 from the cord, a dorsal root, with a ganglion (spinal ganglion) at 

 some distance from the cord, and a ventral root, without a ganglion. 

 The dorsal root contains only sensory fibres i.e., those carrying; 

 nervous impulses to the cord and is afferent, while the ventral 

 roots are efferent and contain only motor elements (Bell's Law). 

 These roots unite into a mixed root, which then divides into dorsal 

 and ventral branches. 



The brain of vertebrates in general corresponds in its funda- 

 mental plan (fig. 568), best seen in development, with the brain of 

 man. At an early stage it consists of three 

 vesicles, one after the other, a fore brain 

 (prosencephalon), a mid brain (mesencepha- 

 lon), and a hind brain (metencephalon). 

 Usually this stage is reached before the 

 closure of the medullary folds. Formerly it 

 was stated that a condition with five vesicles 



-ir 



FIG. 568. FIG. 569. 



FIG. 568. Diagram of a vertebrate brain. (From Wiedersheim.) Aq, aqueduct ; O, 



central canal ; FM, foramen of Monro (connexion of lateral ventricles with each 



other and with the third) ; HH : cerebellum ; MH, corpora bigemina (optic lobes) ; 



NH t medulla oblongata; J?, spinal cord; <SF, lateral ventricles; VH, cerebrum; 



ZH, optic thalami ('twixt brain) ; ///. IF, third and fourth ventricles. 

 FIG. 569. Scheme of brain in sagittal section, c, cerebrum ; c-6, cerebellum; rr, canal 



of spinal cord ; c/i, notochord ; c-s corpus striatum ; /i, hypophysis ; *', infundibu- 



lum ; m, medullary region ; o, optic chiasma ; t>/, olfactory lobe ; oZ, optic lobes ; 



p, pinealis. 



followed upon this with three, the mid brain remaining undivided, 

 while the hind brain divides into cerebellum (cb) and medulla 

 oblongata (m) ; the fore brain into cerebrum and 'twixt brain. 

 This is unnatural so far as the hind brain is concerned, for cere- 



