THE FUNCTIONS OF ORGANIZED CELLS. 23 



structures, the skin and the central nervous system ; from the mesoblast 

 is derived the flesh or muscles of the body, and from the hypoblast the 

 epithelium of the alimentary canal and some of the chief glands, and so on. 



It is obvious that the tissues and organs so derived exhibit in a vary- 

 ing degree the primary properties of protoplasm. The muscles, for 

 example, derived from certain cells of the mesoblast are particularly con- 

 tractile and respond to stimuli readily, while the cells of the liver, 

 although possibly contractile to a certain extent, have to do chiefly with 

 the processes of nutrition. 



Thus, in development, we see that as the cells of the embryo in- 

 crease in number it speedily becomes necessary for the organism to 

 depute to different groups of cells, or to their equivalents (i.e., to the 

 tissues or organs to which they give rise), special functions, so that the 

 various functions which the original cell may be supposed to discharge, 

 and the various properties it may be supposed to possess, become divided 

 up among various groups of resulting cells. The work of each group 

 is specialized. As a result of this division of labor, as it may be called, 

 these functions and properties are, as might be expected, developed and 

 made more perfect, while the tissues and organs arising from each 

 group of cells are developed also, with a view to the more convenient 

 and effective exercise of their functions and employment of their prop- 

 erties. 



In studying the functions of the human body it is necessary first of 

 all to know of what it is composed, of what tissues and organs it is made 

 up; this can of course only be ascertained by the dissection of the dead 

 body, and thus it comes that Anatomy (dvar^uvw, to cut up) the science 

 which treats of the structure of organized bodies, is closely associated 

 with physiology; so closely, indeed, that Histology (l<rro<;, a web), which 

 is especially concerned with the minute or microscopic structure of the 

 tissues and organs of the body, and which is strictly speaking a depart- 

 ment of anatomy, is usually included in works on physiology. There is 

 much to be said in favor of such an arrangement, since it is impossible 

 to consider the changes which take place in any tissue during life, 

 apart from the knowledge of the structure of the tissues themselves. 

 To understand the structure of the human body in an intelligent way, 

 much help is obtained from the study of the structure of other animals, 

 from the lowest to the highest, which is the province of Comparative 

 Anatomy ; while Embryology, which is concerned with the mode of 

 origin of the various tissues in the embryo of each animal, and which is 

 usually studied at the end of physiology, should from some points of 

 view be considered as an introduction to the subject. 



A second important essential to the right comprehension of the 

 changes which take place in the living organism is a knowledge of the 

 chemical composition of the body. Here, however, we can only deal 



