6 THE HUMAN BODY 



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' is the dorsal or neural 

 cavity, and 6 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 

 cavity; / is the alimentary canal opening in front through the 

 nose, i, and mouth, o; h is the heart, Z, a lung, s the sympathetic 

 nervous system, and k a kidney. 



A transverse section through the chest is represented by the 

 diagram Fig. 3, where x is the neural canal containing the spinal 



cord. In the thoracic cavity are 

 seen the heart, h, the lungs, II, 

 part of the alimentary canal, 

 a, and the sympathetic nerve- 

 centers, sy; the dotted line on 

 each side covering the inside of 

 the chest-wall and the outside of 

 the lung represents the pleura. 



Sections through correspond- 



aos th^dv^Tchest regS? mg parts of any other Mammal 

 x, the dorsal tube which contains the wou id agree in all essential points 



spinal cord; the black mass surround- f . . 



ing it is a vertebra; a, the gullet, a part with those represented in FigS. 2 



of the alimentary canal; h, the heart; , 



sy, sympathetic nervous system- II, and o. 



lungs; the dotted lines around them Th* T imhc TVio limh<3 r>rpspnt 



are the leurse rr ribs st the breast- ine ^H" 08 - 



are the pleurse; rr, ribs; st, the breast- 



bone - no such arrangement of cavities 



on each side of a bony axis as is seen in the trunk. They have an 

 axis formed at different parts of one or more bones (as seen at U 



