GENERAL STRUCTURE AND COMPOSITION. 



O 



cavity, and a posterior or abdominal cavity. The alimentary 

 canal and whatever else passes from one of these cavities to 

 the other must therefore perforate 

 the diaphragm. 



In the chest, besides part of the 

 alimentary canal, lie important or- 

 gans, the hearty h, and lungs, lu; 

 the heart being on the ventral side 

 of the alimentary canal. The ab- 

 dominal cavity is mainly occupied by 

 the alimentary canal and organs con- 

 nected with it and concerned in the 

 digestion of food, as the stomach, 

 ma, the liver, le, the pancreas, and 

 the intestines. Among the other 

 more prominent organs in it are the 

 kidneys and the spleen. 



In the dorsal or neural cavity lie 

 the brain and spinal cord, the former 

 occupying its anterior enlargement 

 in the head. Brain and spinal cord 

 together form the cerebro-spinal 

 nervous centre; in addition to this 

 there are found in the ventral cavity 

 a number of small nerve-centres 

 united together by connecting cords, 

 and with their offshoots forming the 

 sympathetic nervous system. 



The walls of the three main cavi- 



ties are lined by Smooth, moist the brain; ee, vertebrae form- 



. ing the solid partition between 



SerOUS membranes. That lining the the dorsal and ventral cavities; 



. ,, , . , ,, . &, the pleural, and c, the abdom- 



QOrsal Cavity IS the arachnoid; that inal division of the ventral cav- 



-, . . i J.T 7 i. il y separated from one another 



lining the chest the pleura; that by the diaphragm, d\ i, the 



Fm . 2 ._D ia grammatic 

 the'neuSfSe?^ 



lining the abdomen the peritoneum; 

 the abdominal cavity is in conse- 

 quence often called the peritoneal 



pavitv T^Yfprrmllv flip walls nf fTioco nervous chain. From the stom- 

 .avity. Externally t nese ach, /, the intestinal tube leads 



cavities are covered by the skin, through the abdominal cavity 



* ' to the posterior opening of the 



Which Consists Of two layers : 'an Outer alimentary canal. 



horny layer called the epidermis, which is constantly being 

 shed on the surface and renewed from below; and a deeper 

 layer, called the dermis and containing blood, which the 



