4 INTRODUCTORY fcH. 1. 



could possibly have been gained by mere cogitation. Many experi- 

 ments involve the use of living animals, but the discovery of anaes- 

 thetics, which renders such experiments painless, has got rid of any 

 objection to experiments on the score of pain. 



We must next proceed to an examination of the general structure 

 of the body, and an explanation of some of the technical terms which 

 will frequently be used hereafter. 



The adult body consists of a great number of different parts; and 

 each part has its own special work to do. Such parts of the body are 

 callecl organs. Each organ does not only its own special work, but 

 acts in harmony with other organs. This relationship between the 

 organs enables us to group them together into what are termed 

 systems. Thus, we have the circulatory system, that is, the group of 

 organs (heart, arteries, veins, etc.) concerned in the circulation of the 

 blood ; the respiratory system, that is, the group of organs (air 

 passages, lungs, etc.) concerned in the act of breathing ; the digestive 

 system, which deals with the digestion of food ; the excretory system, 

 with the getting rid of waste products ; the muscular system, with 

 movement; and the skeletal system, with the support of the softer 

 parts of the body. Over and above all these is the nervous system 

 (brain, spinal cord, nerves), the great master system of the body 

 which presides over, controls, and regulates the functions of the 

 other systems. 



If we proceed still further on our anatomical analysis, and take 

 any organ, we see that it consists of various textures, or, as they are 

 called, elementary tissues. Just as one's garments are made up of 

 textures (cloth, lining, buttons, etc.), so each organ is composed of 

 corresponding tissues. The elementary tissues come under the 

 following four headings: 



1. Epithelial tissues. 3.. Muscular tissues. 



2. Connective tissues. 4. Nervous tissues. 



Each of these is again divisible into sub-groups. 



Suppose we continue our anatomical analysis still further, we find 

 that the individual tissues are built up of structures which require 

 the microscope for their accurate study. Just as the textures of a 

 garment are made up of threads of various kinds, so also in many of 

 the animal tissues we find threads or fibres, as they are called. But 

 more important than the threads are little masses of living material. 

 Just as the wall of a house is made up of bricks united by cement, so 

 the body walls are built of extremely minute living bricks, united 

 together by different amounts of cementing material. Each one of 

 these living units is called a cell. 



Some of the tissues already enumerated consist of cells with only 

 very little cement material binding them together ; this, for instance, 



