PHYSIOLOGY OF THE NEKVOUS SYSTEM. 



769 



In vertebrates the nervous system has reached a much higher stage 

 of development, and here is placed above the digestive canal, in contra- 

 distinction to the position which it occupies when present in the inverte- 

 brates, and is usually inclosed within a bony or cartilaginous cavity ; it 

 is divided into a cephalic portion, confined within the cranium, connected 

 to a long trunk of nerve-cells inclosed within the vertebral canal, con- 

 stituting the spinal cord (Fig. 324). From this central nervous system, 



B 



FIG. 325. BRAIN OF PERCH, AFTER CUVIER. (Rymer 

 Jones. ) 



A, cerebellum; B, cerebrum : C. olfactory ganglion; i, olfactory nerves; 

 D, optic ganglion; G, supplementary lobe; H, transverse fibres in the walls of 

 the cerebral ventricle; N, commissure of the optic nerves; P, Q, R, S, T, U, 

 the third, fourth, fifth, sixth, seventh, and eighth pair of cerebral nerves. 



RH. 



VIII. 



FIG. 324. BRAIN AND 

 SPINAL CORD OF MAN. 

 (Carpenter.) 



FIG. 326. BRAIN OF FROG SEEN FROM ABOVE. (JVv/w.) 



L. OLF, olfactory lobes ; L.H, hemispherical lobe (fore-brain) ; VIII, lobe 

 of the third ventricle; L.O, optic lobes (mid-brain) ; CELL, cerebellum (hind- 

 brain) ; OBL, medulla oblongata ; RH, rhomboidal sinus. 



both from the brain and spinal cord, originate series of fibres which 

 fulfill the functions of conduction of both motor and sensory impulses 

 and which extend to all parts of the body. In connection with this sys- 

 tem, which is spoken of as the cerebro-spinal system, there is usually found 

 a more or less independent series of ganglia, which constitutes the 

 sympathetic system. 



While such a cerebro-spinal system characterizes all vertebrates, it is 



49 



