ir. VEliTEBRATA. 



The gray matter surroiinds tlio 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 

 (iig. 70, ////) 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 iilan (fig. 508), 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 Irve vesicles 



i-sr 



Fig 



Fir.. .%a. 



Fig o6h -Dia"rain of a vertfhrate brain. (From Wiedershcim.) .Ir,, aqucdiirt ; CV, 

 central carnal FM. foramen f,f Monro (connexion of lateral ventric es with each 

 fjther and wit 1 the thirdi ; HH, cerebellum ; MH, corpora bigemina (rjptic lobes) ; 

 'wfmedulTa oblongata; «, .pinal cord; ,SIf, lateral V';"t".<'l«^ ■. .l-f cerebrum; 

 Zli. optic thalami ('twixt braini ; 111.11 , third and fourth ^ '-.nt"tle^^ 



Fig. .-iGS.-Scheme of brain in sagittal section, c, cerebru n ' ■ ' '^,^!',^'.'""| ■ 1^ ',i^?,f^ 

 of spinal cord : c/,, notochord ; cs corpus st""*^'' f^;. J,^^' ,V^'V( intic "X^^ 

 lum ; .11, medullary region ; <-, optic chiasma ; u/, olfactoo lobe , ot, optic loijes , 



p^ pinealis. 



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

 while the hiud brain divides into cerebellum {rh) and medulla 

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

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



