182 MANUAL OF ANIMAL BIOLOGY 



lying between the body wall and alimentary canal, and containing 

 the vascular, excretory, reproductive, and nervous systems. (W. 

 fs. 60, 94, 95.) 



Fundamentally all the Vertebrates have this same general 

 arrangement of the internal organs. Underneath the skin is the 

 body wall, composed largely of muscle tissue or ' flesh.' The body 

 wall of the Vertebrate is much thicker on the dorsal surface than 

 it is on the ventral, but in all regions it forms a continuous and 

 fairly rigid layer. Most of the highly developed organ systems, 

 such as the vascular, excretory, and nutritive systems, are chiefly 

 located in the coelom of the trunk region with extensions to the 

 peripheral regions of the body. The vertebrate coelom, however, 

 consists either of one or two large cavities as compared with many 

 chambers in the Earthworm. ( W. fs. 106-109.) 



We may now proceed to a consideration of the internal structure 

 of the Frog. In order to do this it will be necessary to lay open 

 the ventral body wall of the trunk region by a median cut, which 

 should extend anteriorly to the girdle, or sternum, of the fore 

 limbs and posteriorly to the pelvic girdle of the hind limbs. 

 After cutting through the loose outer layer of skin, the structure of 

 which has been noted above, one comes into contact with the 

 muscular body wall. Cutting through this layer opens up the body 

 cavity, or coelom, which is a single large chamber in the Frog, 

 and thus exposes a number of the most important organs of the 

 body, collectively spoken of as the viscera, which lie in it. Dorsal 

 to the coelom lies the vertebral column, in a specialized portion 

 of which is the central nervous system. This arrangement of 

 the central nervous system, which is typical of the Vertebrates, is 

 very different from that found in the higher Invertebrates in which 

 the nerve cord lies directly in the coelom, close to the ventral body 

 wall. (W. f. 107. 1 ) 



The general arrangement of the organs in the coelom is quite 

 uniform throughout the Vertebrate series, but in the Mammals, 

 including Man, the cavity of the coelom is completely divided by a 

 sheet of muscular tissue, the diaphragm, into an anterior portion, 

 or thoracic cavity, in which are the heart, lungs, and esophagus, 

 and a posterior portion, or abdominal cavity, which contains 

 the remainder of the viscera. ( W. fs. 108, 109.) 



1 In this figure the internal structure of the Frog is viewed from the left side in- 

 stead of from the ventral surface. 



