18 THE HORSE IN HEALTH AND DISEASE 



Histology is a description of the minute structures of the body, 

 and includes a microscopic study of the cells and tissues of which 

 the body is composed. 



Physiology deals with the functions of the different organs in 

 the living body. For example, the function of the liver is to secrete 

 bile; the udder, milk; the kidneys, urine. 



Pathology describes the derangements of the different parts 

 and organs due to diseases. Until we become familiar with nor- 

 mal body processes, pathologic changes cannot be recognized, so 

 this subject will not be considered until after the preceding have 

 been studied. 



All are more or less conversant with animals as they live, move, 

 feed, grow, and raise their young. Most of us have remained 

 satisfied with such knowledge, and have only a very hazy idea of 

 the wonderfully intricate machinery which lies concealed in the 

 animal body. This is not strange when we consider that for many 

 generations men failed to discover even the fundamental truths of 

 the mystery of life. Anatomy and physiology, as our definitions 

 show, deal respectively with the study of structure and function. 

 It is through these that we will learn something about the complex 

 body of the horse and be better enabled to render intelligent aid 

 to this noble animal when occasion demands. 



The position of the horse in the animal kingdom must first be 

 established. All the domesticated animals in North America 

 belong to the sub-kingdom of vertebrates. The distinguishing 

 feature of this group is the vertebral column or back-bone, made 

 up of segments called vertebra, from which the name of the sub- 

 kingdom is derived. Vertebrates are further characterized by 

 never having more than two pairs of limbs, which are always at- 

 tached to the lower part of the body. 



In fetal hfe the body of the vertebrate is traversed by an un- 

 jointed cartilaginous rod. In the lower vertebrates this rod per- 

 sists throughout life, but in the higher it is replaced by the true 

 vertebrae. 



A cross-section of the body of a vertebrate animal shows two 

 cavities, with the vertebral column lying in the partition between 

 the two. The upper cavity is tube-like in form and contains the 

 spinal cord and brain of the central nervous system. The lower 

 cavity contains the ahmentary canal, heart, lungs, and other soft 

 organs. Invertebrates— insects, for example — have but one cavity 



