CHAPTER I 

 INTRODUCTION CHEMICAL PRELIMINARIES 



\YIIKN the body of an animal is examined it is seen at once 

 to consist of many parts, more or less distinct from one 

 another. These separate parts differ in form, in appear- 

 ance, and in the way they are built up. Such differences are 

 spoken of as differences in structure. The separate parts 

 differ also in what they do, in the purposes they serve, in the 

 use they are to the animal. Such differences arc spoken of 

 as differences in function. Parts that differ from one another 

 in this way in structure and function are called organs. 

 The liver, the stomach, the brain, th'e eyes, are examples of 

 organs. 



When any organ is examined it is found not to be of the 

 same structure throughout its substance, but to consist of parts 

 differing both in the materials of which they consist and in the 

 building up of those materials. These different kinds of struc- 

 tural material are called tissues. Muscular tissue, nervous 

 tissue, bony tissue, are examples of tissues. Some tissues 

 contribute to the structure of several different organs ; mus- 

 cular tissue, for instance, occurs not only in what are specially 

 called muscles, but also in the stomach, the intestine, the 

 bladder, the eye, and in many other organs. When a tissue 

 is examined with a microscope, it, in its turn, is found to 

 consist of a number of units, cells, as they are called, built 

 up together ; and one tissue differs from another in the nature 

 of its cells, and in the way they are connected together, just 



B 



