GENERAL STRUCTURE AND COMPOSITION 







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 hearlj 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 t^te 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- 



tudmal section of the .Body, a, 



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

 , , . enlargement in the skull-cavity 



shoots forming the sympathetic nervous a t a'; N, the spinal cord ; N', 



tiKiievn * ne brain; ee > vertebrae form- 



sysiem. ing the so \{^ partition between 



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

 ties; o, the pleural, and c, the 



are lined by Smooth, moist serous mem- abdominal division of the ven- 



m i , ,. . ,-, ! i tral cavity, separated from one 



branes. That lining the dorsal cavity IS another by the diaphragm, d; i, 



thp nrnrhnnif?- tVmt linintr trip oViPst, the the nasal, and o, the mouth 

 ia, tnat \ 3 chamber, opening behind into 



pleura; that lining the abdomen the the pharynx, from which one 



. . . tube leads to the lungs, Z, and 



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

 consequence often called the peritoneal J^ sympathetic 

 cavity. Externally the walls of the 



* * . 



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



. . terior opening of the alimen- 



consists of two layers: an outer horny tarv canal. 



