152 THE HUMAN SPECIES 



cauda equinaconsistingof spinal 

 nerves which do not immedi- 

 ately leave the vertebral canal 

 but are contained therein for a 

 part of their course (see Fig. 79). 



The Brain. 



Among the Vertebrates, the 

 brains of fishes are of simple 

 structure, especially in the lower 

 orders, the five divisions of the 

 brain corresponding in order 

 to the five primitive cerebral 

 vesicles. In the more highly 

 organised forms a gradual de- 

 velopment of bilateral, sym- 

 metrical arches takes place, par- 

 ticularly in the anterior or first 

 vesicle, and later also in the 

 second and third. The first 

 vesicle (cerebrum) begins to 

 overlap those parts posterior 

 to it, and folds forming in its 

 surface represent the first signs 

 of convolution ; the posterior, 

 or fourth, cerebral vesicle (cere- 

 bellum) develops slowly ; the 

 fifth vesicle (medulla oblon- 

 gata) may be distinguished 

 from the spinal cord by its 

 breadth. 



In the amphibians the cere- 

 bellum is divided laterally into 

 two halves. During the larval 

 state the differentiation of the 

 second and third vesicles com- 

 mences, the latter in the Anura 

 increasing in bulk and dividing 

 into two halves. The cere- 



Trunk of and 

 cervical nerve 



Trunk of ist 

 dorsal nerve 



Trunk of ist 

 lumbar nerve 



Trunk of ist 

 sacral nerve 



FIG. 79. Spinal cord ; front view. 

 One-third of natural size. 

 (Thome', Zoologie.) 



