CELLS AND THEIR ASSOCIATION 49 



upon the contributions of others for its continued 

 existence. 



One of the capacities which may be lost in connection 

 with specialization of structure and function on the part 

 of cells is that of reproduction. We have seen that this 

 is retained by epithelial units; its persistence in the hair 

 and the nails is most remarkable. But it is not retained 

 by the most conspicuous kind of muscle in the body. 

 The enlargement 0f a muscle in consequence of exercise 

 is due to increased size of the individual units and not 

 to their multiplication. So, too, in the nervous system: 

 the wonderful advances in the working of the brain from 

 infancy to maturity are not assisted by the addition of a 

 single cell to the number originally present but only by 

 the organization of this collection. 



Elements of Anatomy 



Our natural interest will center in the physiology of 

 man, and before we go farther the outlines of human 

 anatomy may be very briefly suggested. The skin, 

 which forms the surface of the body, is an epithelium 

 of many layers. Beneath it there is loose connective 

 tissue, more or less rich in fat. Deeper still we find the 

 muscles. The bones, articulated as the skeleton, give 

 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 

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