Chap. XII.] 



BRAIN OF VERTEBRATA. 



CH- 



CV- 



N.. 



-H3 



a constriction from the cord that follows it. and from 

 which three pairs of nerves have been observed to be 

 given off. In this cord, as in that of the Yertebrata, 

 we find that the nerve fibres lie externally to the gang- 

 lionic cells, an arrangement of the histological elements 

 which is exactly the reverse of 

 what obtains in "invertebrates." 



With the loss of the tail, the 

 nerve cord, which is found in the 

 tailed larva in the same position 

 as in the adult Appendicularia, 

 undergoes atrophy, and the fixed 

 or colonial Tunicate has a single 

 ganglionic mass which lies be- 

 tween the mouth and the atrio- 

 pore. (See page 231.) From 

 this ganglion nerves are given 

 off to the different parts of the 

 body. 



In the Vertebrata a brain 

 is always present ; the primi- 

 tively single swelling at the an- 

 terior end rapidly becomes divided 

 into three brain vesicles, which 

 may be distinguished as those of 

 the fore-, mid-, and hind- brain. 

 These vesicles are, of course, hol- 

 low within, and their cavities 

 have received distinct names, the 

 reasons for which will certainly 

 be far from clear, unless we recollect that the termin- 

 ology of the parts of the vertebrate brain is based on 

 the nomenclature of anthropotornists. The cavity in 

 the fore-brain (Fig. 178; m) is known as the third 

 ventricle, and that in the hind-brain (iv) as the 

 fourth ventricle ; the often narrower cavity in the 

 mid -brain (it) is known as the iter a tertio 

 BB 16 



Fig. 178. Diagram of the 

 Ventricles of the 

 Vertebrate Brain. 



in, third ventricle ; it, iter ; 

 iv, fourth ventricle ; CH, 

 cerebral hemispheres; 

 cv, their cavity ; FM, fora- 

 men of Munro ; FB, f ore- 

 brain ; JIB, mid-brain : 

 HB, hind-brain. 



