The Nervous System. 



69 



using spinal or decerebrated frogs, but similar actions, ot which the 

 well-known scratch reaction of the dog is an example, are common 

 to all animals. 



The spinal cord reflects in its form the ground type of the cen- 

 tral nervous system. Developed in the embryo as a tube, it retains 

 this condition throughout life. The cavity, however, is reduced 

 to a slender central canal, lined by a thin epithelial membrane, 

 THE SPINAL T ^ e e P en dy ma > while through proliferation of its 

 CORD cells, and their fibre extensions, the walls become 



enormously thickened, and the nervous functions 

 of the system, as a connected conducting mechanism, thereby 

 established. The cord traverses the vertebral canal, showing slight 



bo 



Fig. 37. The brain from the left side: bo, olfactory bulb; c, cerebellum; 

 fc, flocculus cerebelli; h, cerebral hemisphere; !p. piriform lobe (olfactory 

 brain); m, medulla oblongata; s. spinal cord. Numerals indicate the cor- 

 responding cranial nerves; 2, optic: 4, trochlear; 5, trigeminal; 6, abducens; 

 7. facial; 8. acoustic; 9. glossopharyngeal; 10, vagus; 11, spinal accessory: 12, 

 hypoglossal. 



enlargements in the cervical and lumbar regions in relation to the 

 nerve supply of the limbs, and at about the level of the second 

 sacral vertebra narrows into the slender, thread-like filum ter- 

 minate, by which it is continued almost to the middle of che length 

 of the tail. 



Superficial examination of the brain of the rabbit (Fig. 37) 

 shows that its larger part is formed by the paired cerebral hemis- 

 pheres. They are closely pressed together on the dorsal side 

 but separated posteriorly on the ventral side. The external layer, 

 important as forming the cellular cortex, is smooth in the rabbit 

 as' in rodents generally, but is corrugated in higher mammals such 

 as the cat and dog, while in man it reaches a high degree of elabora- 



