GENERAL STRUCTURE AND COMPOSITION 5 



In the chest, besides part of the alimentary canal, lie important 

 organs, the heart, h, and lungs, lu (Fig. 1) ; 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, 

 st, the liver, li, the pancreas, and the small and large intestines, si 

 and c. Among the other more prominent organs in it are the kid- 

 neys 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 cerebrospinal nervous center com- 

 monly 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 by connect- 

 ing cords, and with their off-shoots forming the sympathetic nervous 

 system. 



The walls of the three main cavities are lined by smooth, 

 moist serous membranes. That lining the dorsal cavity is the 

 arachnoid; that lining the chest the pleura; that lining the abdo- 

 men the peritoneum; the abdominal cavity is in consequence often 

 called the peritoneal cavity. Externally the walls of these cavities 

 are covered by the skin, which consists of two layers : an outer 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 epidermis does not. Be- 

 tween the skin and the lining serous membranes are bones, muscles 

 (the lean of meat), and a great number of other structures which 

 we shall have to consider hereafter. All cavities inside the Body, 

 as the alimentary canal and the air-passages, which open directly 

 or indirectly on the surface are lined by soft and moist prolonga- 

 tions of the skin known as mucous membranes. In these two layers 

 are found as in the skin, but the superficial bloodless one is called 

 epithelium and the deeper vascular one corium. 



Diagrammatically we may represent the Human Body in lon- 

 gitudinal section as in Fig. 2, where aa f is the dorsal or neural 

 cavity, and b and c, respectively, the thoracic and abdominal sub- 

 divisions of the ventral cavity; d represents the diaphragm separat- 

 ing them; ee is the vertebral column with its modified prolongation 

 into the head beneath the anterior enlargement of the dorsal 



