GENERAL STRUCTURE AND COMPOSITION 



O 



anterior chest or thoracic cavity, and a posterior, or abdominal 

 cavity. The alimentary canal and whatever else passes from one 

 of these cavities to the other must there- 

 fore perforate the diaphragm. 



In the chest, besides part of the ali- 

 mentary canal, lie important organs, the 

 heart, h, and lungs, lu; the heart being 

 on the ventral side of the alimentary 

 canal. The abdominal cavity is mainly 

 occupied by the alimentary canal and 

 organs connected with it and concerned 

 in the digestion of food, as the stomach, 

 ma, the liver, le, the pancreas, and the in- 

 testines. Among the other more promi- 

 nent organs in it are the kidneys and the 

 spleen. 



In the dorsal or neural cavity lie the 

 brain and spinal cord, the former occu- 

 pying its anterior enlargement in the 

 head. Brain and spinal cord together 

 form the cerebrospinal nervous center 

 commonly called the central nervous 

 system; in addition to this there are 

 found in the ventral cavity a number 

 of small nerve-centers united to each 

 other and to the cerebrospinal center FIG. 2. Diagrammatic longi- 



f . tudmal section of the Body, a, 



by connecting COrds, and With their oft- the neural tube, with its upper 

 i f ,1 ,T , enlargement in the skull-cavity 



snoots torming the sympathetic nervous a t a'; N, the spinal cord; N', 



System ^ e brain; ee > vertebras form- 



ing the solid partition between 



The walls of the three main cavities the dorsal and ventral cavi- 



T , , . ties; b, the pleural, and c, the 



are lined by smooth, moist SerOUS mem- abdominal division of the ven- 



branes. That lining the dorsal cavity is aiTotheVby 



the arachnoid; that lining the chest the *jj e nasa1 ' and ~> 



, 



. chamber, opening behind into 



that lining the abdomen the the pharynx, from which one 



., 11-1 -A tube leads to the lungs. I. and 



peritoneum; the abdominal cavity is in another to the stomach, /, h, 



pleura; 



consequence often called the peritoneal 



cavity. Externally the walls of these From the stomach, /, the in- 



testinal tube leads through the 



cavities are covered by the skin, which abdominal cavity to the pos- 

 consists of two layers: an outer horny ^rycaTaf ng f the 



