38 THE HUMAN BODY. 



account the arrangement of all these parts within it, and also 

 its connections with other organs of the Body. The physi- 

 ology of any muscle must take into account 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 

 matter. In a factory we frequently find machines arranged 

 so that two or more shall work together for the perform- 

 ance 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 ap- 

 paratuses. 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 according 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 con- 

 tains highly irritable parts; these, changed by the light, 

 stimulate (c) the optic nerve, which is conductive and trans- 

 mits a disturbance which arouses in turn (d) sensory parts in 

 the brain. In the production of ordinary sight sensations all 

 these parts are concerned and work together as a visual appa- 

 ratus. So, too, we find a respiratory apparatus, consisting 

 primarily of two hollow organs, the lungs, which lie in the 

 chest and communicate by the windpipe with the back of the 

 throat, from which air enters them. But to complete the 

 respiratory apparatus are many other organs, bones, muscles, 

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

 air in the lungs from time to time; and the act of breathing 

 is the final result of the activity of the whole apparatus. 



Many similar instances, as the alimentary apparatus, the 



