SUGGESTIONS FOR LABORATORY WORK 597 



the vagus, the inferior laryngeal, passes behind a large artery, the 

 aorta, and turns back toward the head. 



Branches of the vagus can be traced into the root of the lung. 

 At about the level of the root of the lung the nerve divides into two 

 branches, each of which can be traced to the surface of the esoph- 

 agus, where each unites with a corresponding branch from the 

 right vagus. The nerve trunks thus formed continue backward 

 along the esophagus to the stomach where they break up into fine 

 branches which supply the stomach and upper portion of the small 

 intestine. 



For the CENTRAL NERVOUS SYSTEM the sheep's brain is a satisfac- 

 tory object of dissection. The brain should be carefully removed 

 from the skull and hardened in formalin before use. 



Sheep's Brain. With the aid of the figures on pages 144, 145, 

 146, and 151 make out the grand divisions of the brain: cerebrum, 

 or fore brain; midbrain, overlain in front by the pom, and behind 

 by the cerebellum; medulla oblongata, forming the connecting link 

 between brain and spinal cord. 



Observe on the surface of the cerebrum the irregular convolutions 

 which serve to increase its surface relative to its bulk. 



On the base of the brain make out with the aid of the figure on 

 page 151 the optic tracts and optic chiasma; also as many other roots 

 of cranial nerves as possible. Note that all cranial nerves behind 

 the optic nerve spring from the brain stem (midbrain and medulla). 

 In front of the optic tracts and springing from the cerebrum, the 

 olfactory lobes may be seen. 



Cut the brain through the vertical median plane. Note the 

 corpus callosum. 



Conduction in the Nerve Trunk. The sciatic nerve in the frog 

 can be exposed by removing the skin from the leg and separating 

 carefully the two large muscles on the dorsal surface of the thigh. 

 Dissect the nerve out carefully from the upper end of the leg toward 

 the knee. Use great care to avoid injuring the nerve by stretching 

 or squeezing. Cut the nerve away at its upper end. Leave it in 

 connection below with the muscles of the shank. The ability of 

 the nerve to conduct impulses may be demonstrated by stimulating 

 it as far as possible from the muscles and observing their response. 

 The susceptibility of the nerve to different forms of energy may 

 be shown by stimulating it with forceps (mechanical), a hot rod 



