THE NERVOUS SYSTEM 95 



considerable cavity within the optic lobes. 

 It forms a connection between the third and 

 fourth ventricles. The last-named occurs 

 between the medulla oblongata and the cere- 

 bellum and into it opens the minute central 

 canal of the spinal cord. 



Twelve pairs of cerebral nerves take origin 

 from the brain. They are named in the same 

 manner as the mammalian nerves and have 

 essentially the same distribution, namely : — I., 

 olfactory; II., optic; III., oculo -motor ; IV., 

 trochlear; V., trigeminal; YI., abducent; VII., 

 facial ; VIII., acoustic ; IX., glosso-pharyngeal ; 

 X., vagus; XI., accessory; XII., hypoglossal. 



Spinal nerves, each arising by two roots from 

 the spinal cord, agree in name and number 

 with the vertebrae. The dorsal root of each 

 nerve possesses a spinal ganglion as in mammals. 

 The last two or three cervical and the first 

 one or two thoracic nerves join in the formation 

 of a brachial plexus from which the nerves 

 i supplying the wing are derived. The sacral 

 nerves form two plexuses which furnish the 

 nerves of the leg. 



The sympathetic system of nerves consists 

 of a double chain of ganglia lying immediately 

 ventral to the vertebral column. The ganglia 



