ORGANS. 35 



of the individual tissues would be comparable to that at- 

 tained about a great factory by studying separately the 

 boilers, pistons* levers, wheels, etc., found in it, and leav- 

 ing out of account altogether the way in which these are 

 combined to form various machines; for in. the Body the 

 various tissues are for the most part associated to form 

 organs, each organ answering to a complex machine like a 

 steam-engine with its numerous constituent parts. And 

 just as in different machines a cogged wheel may perform 

 very different duties, dependent upon the way in which it 

 is connected with other parts, so in the Body any one tis- 

 sue, although its essential properties are everywhere the 

 same, may by its activity subserve very various uses accord- 

 ing to the manner in which it is combined with others. 

 For example: A nerve-fibre uniting the eye with one part 

 of the brain will, by means of its conductivity, when its 

 end in the eye is excited by the irritable tissue attached to 

 it on which light acts, cause changes in the sensory nerve- 

 cells connected with its other end and so arouse a sight 

 sensation; but an exactly similar nerve-fibre running from 

 the brain to the muscles will, also by virtue of its conduc- 

 tivity, when its ending in the brain is excited by a change 

 in a nerve-cell connected with it, stir up the muscle to con- 

 tract under the control of the will. The different results 

 depend on the different parts connected^with the ends of 

 the nerve-fibres in each case, and not oil any difference in 

 the properties of the nerve-fibres themselves. 



It becomes necessary then to study the arrangement and 

 uses of the tissues as combined to form various organs, and 

 this is frequently far more difficult than to make out the 

 structure and properties of the individual tissues. An or- 

 dinary muscle, such as one sees in the lean of meat, is a 

 very complex organ, containing not only contractile mus- 

 cular tissue, but supporting and uniting connective tissue 

 and conductive nerve-fibres, and in addition a complex 

 commissariat arrangement, composed in its turn of several 

 tissues, concerned in the food supply and waste removal of 

 the whole muscle. The anatomical study of a muscle has 



