140 FOUNDATIONS OF BIOLOGY 



4. Coelom 



The Vertebrate coelom, in contrast with the condition in the 

 Earthworm, essentially comprises only two chambers a large 

 ABDOMINAL cavity which contains most of the chief viscera, 

 and a small, anterior, PERICARDIAL cavity in which the heart 

 is situated. In the Mammals, including Man, however, the 

 anterior chamber, known as the THORAX, contains the heart 

 and lungs and is separated from the abdominal cavity by 

 a muscular partition, or DIAPHRAGM. The lining membrane 

 of the coelom is known as the PERITONEUM and forms the 

 innermost layer of the body wall. (Figs. 74, 82-87.) 



5. Skeleton 



The form of the Vertebrate body is maintained by a system 

 of supporting and protecting structures, termed the SKELE- 

 TON. Although various outgrowths of the skin, such as scales, 

 feathers, and hair, form a part of the skeletal system known as 

 the EXOSKELETON which is comparable to the protective 

 coverings of the Invertebrates, it is a bony ENDOSKELETON 

 which is characteristic of the higher animals. This internal 

 skeleton which is largely mesodermal in origin exhibits 

 such great diversity and complexity that its study, known as 

 OSTEOLOGY, forms a most important subdivision of compara- 

 tive anatomy. In the lower Fishes the endoskeleton is com- 

 posed of a firm elastic tissue, CARTILAGE, or gristle, but from 

 the 'bony' Fishes to Man most of the cartilage becomes ossi- 

 fied: that is, impregnated with lime salts and transformed 

 into BONE. The human skeleton is formed of about 200 

 separate bones, but the number varies at different periods 

 of life, because some bones which at first are distinct later 

 become fused. (Figs. 77, 81, 186.) 



While it is true that the bones constitute the main support- 



