50 HUMAN PHYSIOLOGY 



use of the word needs to be carefully defined. The 

 student is apt to imagine that the so-called body cavities 

 contain more or less vacant space. This is not the actual 

 condition. They are only potential cavities which be- 

 come real ones when their contents have been removed. 

 In life they are completely filled by the organs, plus a 

 small quantity of fluid. 



In the head the principal cavity is that which we call 

 the cranial one, the space which accommodates the brain. 

 It is bounded by bones pierced here and there by small 

 openings for the nerves and at one place by the larger 

 orifice through which the spinal cord descends. This 

 last-mentioned opening is at the base of the skull and 

 leads to a tunnel made by the successive bony arches 

 of the vertebrae. In the trunk we distinguish two main 

 cavities, that of the thorax within the sweep of the ribs 

 and that of the abdomen below. The partition between 

 them is the diaphragm, a sheet composed partly of 

 muscular and partly of connective tissue which has the 

 form of a tolerably high dome and therefore subtracts 

 much space from the apparent size of the thorax and adds 

 it- to the abdomen. Below the abdomen and within the 

 circle of the hip-girdle is the small pelvic cavity. 



A striking fact about the body cavities is that the 

 organs which they contain are not attached to the en- 

 compassing walls save at a few places. For the most 

 part the surfaces of the organs bear upon surfaces of 

 the body walls but do not adhere to them. The arrange- 

 ment permits a certain amount of gliding of one upon 

 the other. There cannot normally be a separation be- 

 tween them since this would involve the creation of a 

 vacuum. The relation between an organ and the 

 opposing body wall is much like that between two 

 plates of glass which have been moistened and laid 

 together. One will slide freely upon the other, but it 

 requires great force to pull them apart until air begins 

 to penetrate between them. 



The thorax is divided vertically into right and left 



