Branch Chordata 97 



the spinal cord, and is now wholly invisible within the 

 cranium and the spinal column. The sympathetic sys- 

 tem may be seen in the shape of white cords lying 

 on the dorsal floor of the body wall on each side of the 

 spinal column. It is connected with the cerebrospinal 

 system by branches between the vertebrae. The func- 

 tion of the cerebrospinal system is to govern the 

 action of the muscles attached to the skeleton (skele- 

 tal muscles), while the sympathetic system controls 

 the action of the visceral organs. By bending, sever the 

 connection between the vertebrae, in order to see the 

 spinal cord lying in a cavity within the solid bone of 

 the spinal column. Carefully remove the bone from 

 the dorsal surface of the cranium to see how the spinal 

 cord terminates in the brain. 



The position of the central nervous system (cerebro- 

 spinal) lying in a tube parallel to and wholly separated 

 from the main tube (body cavity), and on the dorsal 

 side of the body is an important characteristic of verte- 

 brates as distinguished from invertebrates. 



XIV. — Study again the pectoral arch, — the circlet of 

 bones supporting the forward legs. It corresponds 

 to our collar bone and shoulder blade. 



XV. — Study the pelvic arch — the bones supporting the 

 hind legs. Study the bones of the fore and hind limbs, 

 noticing the similarity between the two pairs in the ar- 

 rangement of the bones. Make comparisons between 

 the bones of the hmbs of the frog and those of the 

 higher vertebrates by aid of the following outline : — 



In the fore legs, beginning with the segments next to 

 the body, the names and homologies are : (a) humerus, 

 which corresponds to the upper arm in man ; (3) ulno- 

 radius, which corresponds to the ulna and radius of our 



