2 TEXT-BOOK OF PHYSIOLOGY 



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 a given 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. 



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 inter- 

 nal 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, constitutes a department of anatomic science known as histology, or 

 as it is prosecuted largely with the microscope, microscopic anatomy. 



Anatomy may naturally be divided into: 



1. Individual anatomy, the object of which is the investigation of the construc- 



tion, 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. 



Human anatomy is that department of anatomic science which has 

 for its object the investigation of the construction of the human body. 



GENERAL STRUCTURE OF THE ANIMAL BODY 



The body of every animal, from fish to man, may be divided into 



1. An axial portion, consisting of the head, neck, and trunk; and 



2. An appendicular portion, consisting of the anterior and posterior limbs 

 or extremities. 



The Axial Portion. The axial portion of all mammals, to which class 

 man zoologically belongs, as well as of all birds, reptiles, amphibians, and 

 osseous fish, is characterized by the presence of a bony, segmented axis, 

 which extends in a longitudinal direction from before backward, and which 

 is known as the vertebral column or backbone. In virtue of the existence 

 of this column all the classes of animals just mentioned form one great 

 division of the animal kingdom, the Vertebrata. 



Each segment, or vertebra, of this axis consists of 

 i. A solid portion, known as the body or centrum, and 



