120 VERTEBRATA. 



cord. The lower Vertebrates with cold blood have a relatively 

 small brain, the mass of which is still considerably smaller than that 

 of the spinal cord. In the warm-blooded Vertebrates, on the other 

 hand, this proportion is reversed, and the more markedly, the higher 

 the organisation and grade of life of the animal in question. 



The spinal nerves arise in pairs from the spinal cord : each nerve 

 has two roots a dorsal sensory root and a ventral motor root. They 

 correspond in number with the vertebrae, between which they pass 

 out, so that the spinal cord repeats in a general manner the seg- 

 mentation of the vertebral column. 



In the brain the arrangement of the nerves presents several com- 

 plications which are further increased by the origin of two sensory 

 nerves the olfactory and optic. In spite of the differences in form 



FIG. 677. a. Brain and anterior part of the spinal cord of a human embryo seen from the 

 side (after Kolliker). Vh, Fore-brain; Zh, thalamencephalon ; Mh, mid-brain : Hh, 

 hind-brain ; Nh, medulla oblongata ; T, anterior ventral end of the thalamencephalon ; 

 NO, optic nerve. 6. Diagrammatic longitudinal section through a vertebrate brain (after 

 Huxley). Hs, Hemispheres ; , LO, olfactory lobes ; Olf, olfactory nerve ; TkO, optic 

 thalamus ; Vt, third ventricle ; No, optic nerve ; H, pituitary body (hypophysis) ; Gp, 

 pineal gland ; CQ, corpora quadrigemina ; Cb, cerebellum ; MO, medulla oblongata 

 PV, pons Varolii. 



and structure presented by the brain, three principal regions which 

 correspond to the three vesicles found in the embryo can always be 

 distinguished (fig. 576). The anterior vesicle (fore-brain, fig. 576, Vh) 

 corresponds to the cerebral hemispheres and the optic thalami (fig. 577, 

 Us, ThO), the middle vesicle (mid-brain, Mh) to the corpora quadri- 

 gemina (fig. 577, C Q), and the posterior vesicle (hind-brain, fig. 576, 

 Hh) to the cerebellum and medulla oblongata (fig. 577, Cb, MO). The 

 anterior vesicle, however, is again divided into two parts an anterior 

 bilobed part, which constitutes the cej ebral hemispheres and contains 

 the lateral ventricles, and a posterior unpaired part which constitutes 

 the so-called thalamencephalon with the thalami optici and the parts 

 surrounding the third ventricle (fig. 577). The third cerebral vesicle 

 is also divided into two parts anteriorly the cerebellum, and pos- 

 teriorly the medulla oblongata. 



