TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 35 



fibers in each case, and not on differences in the properties of the 

 nerve-fibers themselves. 



It becomes necessary then to study the arrangement and uses 

 of the tissues as combined to form various organs, and this is fre- 

 quently far more difficult than to make out the structure and 

 properties of the individual tissues. An ordinary muscle, such as 

 one sees in the lean of meat, is a very complex organ, containing 

 not only contractile muscular tissue, but supporting and uniting 

 connective tissue and conductive nerve-fibers, and in addition a 

 complex commissariat arrangement, composed in its turn of sev- 

 eral tissues, concerned in the food-supply and waste-removal of the 

 whole muscle. The anatomical study of a muscle has 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 account the actions of all these parts work- 

 ing together and not merely the functions of the muscular fibers 

 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 Systems. Even the study of organs added to 

 that of the separate tissues does not exhaust the matter. In a fac- 

 tory 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 to- 

 gether 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 physi- 

 ological apparatuses or systems. The circulatory system, for ex- 

 ample, 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 

 (6) 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 in turn (d) sensory parts in 



