THE FROG. 185 



hyoid cartilage. Lay open the anterior part of the trachea, 

 and note the structure of its walls. 



Again inflate the lungs through the trachea, and study 

 their structure. Observe that they are composed of rela- 

 tively few and large air cells (or, more properly, air 

 spaces), and that the walls of these do not afford a very 

 great area over which the blood can be distributed for 

 aeration. The distribution of the capillaries in the skin 

 is such as to bring the blood near enough to the surface 

 for partial aeration there. But the total supply of oxy- 

 gen obtained from both sources is relatively small, and 

 would be inadequate to any but a cold-blooded, sluggish 

 animal. 



The Nervous System. Turn the organs of the body 

 cavity gently to one side until the spinal column is 

 visible. Observe the white spinal nerves extending out 

 from it, along the body wall, just beneath the smooth, 

 transparent peritoneum. Observe that each spinal nerve, 

 near its origin, sends a branch ventrally into the 

 body cavity to meet a very delicate nerve cord that ex- 

 tends longitudinally, ventral to the spinal column, and 

 near the median plane of the body. Examine this nerve 

 cord carefully with a lens to find minute ganglionic swell- 

 ings on it at its junctions with the branches of the spinal 

 nerves. Find also minute nerves arising from its gan- 

 glia, and extending to the internal organs. Trace it for- 

 ward to the head. Find another similar nerve cord on the 

 other side of the median plane. Find minute white com- 

 missural nerves connecting the ganglia of the two chains. 



This double series of ganglia, with connecting commis- 

 sures and radiating fibers, included within the body cavity 

 and distributed to the nutritive organs, constitutes the 

 sympathetic system. The cerebro-spinal system consists of 

 brain and spinal column and radiating nerves. 



