THE NERVOUS SYSTEM. 69 
using spinal or decerebrated frogs, but similar actions, of 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 the ependyma, 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 

Fic. 37. The brain from the left side: bo, olfactory bulb; c¢, cerebellum; 
fc, paraflocculus cerebelli; h, cerebral hemisphere; 1p, 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; I1, 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- 
minale, by which it is continued almost to the middle of the 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- 
