36 THE HUMAN BODY* 



to take into account the arrangement of all these parts 

 within it, and also its connections with other organs of the- 

 Body. The physiology of any muscle must take into ac- 

 count the actions of all these parts working together and 

 not merely the functions of the muscular fibres themselves, 

 and has also to make out under what conditions the muscle- 

 is excited to activity by changes in other organs, and what 

 changes in these it brings about when it works. 



Physiological Mechanisms. Even the study of organs 

 added to that of the separate tissues does not exhaust the 

 whole matter. In a factory we frequently find machines 

 arranged so that two or more shall work together for the 

 performance of some one work : a steam-engine and a loom 

 may, for example, be connected and used together to weave 

 carpets. Similarly in the Body several organs are often 

 arranged to work together so as to attain some one end by 

 their united actions. Such combinations are known as 

 physiological apparatuses. The circulatory apparatus, for 

 example, consists of various organs (each in turn composed 

 of several tissues) known as heart, arteries, capillaries, and 

 veins. The heart forms a force-pump by which the blood 

 is kept flowing through the whole mechanism, and the 

 rest, known together as the blood-vessels, distribute the 

 blood to the various organs and regulate the supply accord- 

 ing to their needs. Again, in the visual apparatus we find 

 the co-operation of (a) a set of optical instruments which 

 bring the light proceeding from external objects to a focus 

 upon (b) the retina, which contains highly irritable parts; 

 these, changed by the light, stimulate (c) the optic nerve, 

 which is conductive and transmits a disturbance which 

 arouses finally (d) sensory parts in the brain. In the pro- 

 duction of ordinary sight sensations all these parts are con- 

 cerned and work together as a visual apparatus. So, too, 

 we find a respiratory apparatus, consisting primarily of two 

 hollow organs, the lungs, which lie in the chest and com- 

 municate by the windpipe with the back of the throat, 

 from which air enters them. But to complete the respi- 

 ratory apparatus are many other organs, bones, muscles, 

 nerves, and nerve-centres, which work together to renew 



