CELLS AND THEIR ASSOCIATION 49 



support and fixed proportions to the whole. In various 

 places between the skin and the bones we find the white 

 nerves and the large blood-vessels. These are of two 

 classes, arteries which carry blood away from the heart 

 and veins whicn conduct blood back toward that organ. 

 The arteries generally lie at some depth below the surface 

 while veins are situated at all depths, the superficial ones 

 being visible through the skin. 



The Body Cavities. The features which have been 

 mentioned are all that need be included in a simple 

 description of what is to be seen in dissecting an arm 

 or a leg, but the case is different with the head or the 

 trunk. In these are what we speak of as cavities; the 

 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- 



