i8 TEXT-BOOK OF PHYSIOLOGY. 



2. Comparative physiology, the object of which is a comparison of 

 the vital phenomena or functions exhibited by the organs of 

 two or more animals of different species, with a view to un- 

 folding their points of resemblance or dissimilarity. 



/ Human physiology is that department of physiologic science 

 [ which has for its object the study of the functions of the organs of 

 \ the human body in a state of health. 



Inasmuch as the study of function, or physiology, is associated 

 with and dependent on a knowledge of structure, or anatomy, it is 

 essential that the student should have a general acquaintance not 

 only with the structure of man, but with that of typical forms of 

 lower animal life as well. 



If the body of any animal be dissected, it will be found to be 

 composed of a number of well-defined structures, such as heart, 

 lungs, stomach, brain, eye, etc., to which the term organ was originally 

 applied, for the reason that they were supposed to be instruments 

 capable of performing some important act or function in the general 

 activities of the body. Though the term organ is usually employed 

 to designate the larger and more familiar structures just mentioned, 

 it is equally applicable to a large number of other structures which, 

 though possibly less obvious, are equally important in maintaining 

 the life of the individual e. g., bones, muscles, nerves, skin, teeth, 

 glands, blood-vessels, etc. Indeed, any complexly organized struc- 

 ture capable of performing some function may be described as an 

 organ. A description of the various organs which make up the body 

 of an animal, their external form, their internal arrangement, their 

 relations to one another, constitutes the science of animal anatomy. 



This may naturally be divided into: 



1. Special anatomy, the object of which is the investigation of the 



construction, form, and arrangement of the organs of any 

 individual animal. 



2. Comparative anatomy, the object of which is a comparison of the 



organs of two or more animals of different species, with a view 

 to determining their points of resemblance or dissimilarity. 

 If the organs, however, are subjected to a further analysis, they 

 can be resolved into simple structures, apparently homogeneous, to 

 which the name tissue has been given e. g., epithelial, connective, 

 muscle, and nerve tissue. When the tissues are subjected to a 

 microscopic analysis, it is found that they are not homogeneous 

 in structure, but composed of still simpler elements, termed cells 

 and fibers. The investigation of the internal structure of the organs, 

 the physical properties and structure of the tissues, as well as the 

 structure of their component elements, the cells and fibers, con- 

 stitutes a department of anatomic science known as histology, 



