2 ELEMENTARY PHYSIOLOGY 



which has its own particular function to perform. These parts 

 are called organs. Thus, we speak of the heart as the chief organ 

 of circulation, the stomach as an organ of digestion, the lungs as 

 organs of respiration, and the muscles as the organs for producing 

 motion. 



The various organs of the body may be conveniently arranged 

 in groups or systems, according to their respective functions. 

 The chief of these systems are : 



i. The osseous, or bony system. 



2 The muscular system. 



3. The digestive system. 



4. The absorptive system. 



5. The circulatory system. 



6. The respiratory, or breathing system. 



7. The excretory, or purifying system. 



8. The nervous system. 



All the organs of the body are more or less complicated in 

 structure. Each one is built up of elementary structures which 

 are called tissues. For example, the heart is composed of a fleshy 

 substance which we term muscular tissue, some fatty substance or 

 adipose tissue, nervous tissue, and a certain amount of connective 

 tissue. 



The osseous (Lat. os, a bone) system consists of a large number 

 of bones, which constitute the skeleton, and form a strong frame- 

 work, which supports and protects the softer structures of the body. 

 These bones are connected together in such a manner as to form 

 joints or articulations (Lat. articulus, a joint), and are bound 

 firmly together at these joints by strong white fibrous bands called 

 ligaments (Lat. ligo, I bind). In some parts of the body, where 

 an elastic and yielding substance is required, which is at the same 

 time very strong, cartilage or gristle takes the place of bone, as in 

 the more prominent part of the nose. 



We may regard the backbone as the central portion of the 

 osseous system, for a glance at the accompanying figure will show 

 that the other bones of the skeleton are connected either directly 

 or indirectly with it. This bone (or, rather, column of bones) 

 passes through the hinder part of the trunk of the body, part of it 

 belonging to the neck, part to the back, and part to the loins. 

 The ribs are connected with that portion which lies in the back, 

 and, extending round the chest, are for the most part connected 

 with the breast-bone in front. At the upper part of the chest are 

 the shoulder-bones : to these the bones of the arms are attached. 

 The lower portion of the backbone is wedged in between the 

 bones of the hip, which form a hollow basin-like cavity, and 

 support the organs of the abdomen. 



