CHAPTER XIV 

 HISTOLOGY AND CYTOLOGY 



IN the foregoing pages we have seen that the body of a higher 

 animal consists of a supporting skeleton, the flesh, and a large 

 number of organs arranged in systems ; we cam even speak of the 

 skeletal and muscular systems regarding the individual parts as 

 organs. Generally speaking, each of these organs is of a character- 

 istic shape, and always, when examined in greater detail, is seen to 

 be composed of a definite tissue, as it is termed, or a combination of 

 tissues. When we proceed still further we find that many of these 

 tissues are composed of small units, termed cells, and when we take 

 into account their development we find that all parts of the body are 

 composed of cells or of structures formed by or from cells. This 

 statement of the composition of the body of the higher animals is 

 sometimes termed the " cell theory," and while it is possible to 

 indicate certain exceptions, it may be taken to be in the main a true 

 one. We thus have two special branches of minute anatomy, and 

 also of physiology, since we can also discuss them from the point 

 of view of function, namely, " Histology," the study of the tissues, 

 and " Cytology," the study of the cell. Certain general points 

 regarding each of these -need considering. 



Histology. 



The actual details of the histology of a number of different 

 tissues have already been dealt with in treating the different animal 

 types, and it now only remains to indicate the way in which these 

 tissues may be classified for the purposes of study. In the first 

 place we may regard them from the point of view of similarity of 

 structure and function, and we find that they can be separated into 

 four main groups : (i) Epithelial, (2) Connective, (3) Muscular, and 

 (4) Nervous tissue. In addition to these are the two body fluids, 

 the blood and the lymph, which, since they contain cells, may be 

 conveniently treated with the tissues, and, indeed, are regarded by 

 some authorities as being highly specialised connective tissue. 



An epithelium is a layer of cells covering free surface ; it need not 



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