THE DIFFERENTIATION OF TISSUES. 37 



ture, and physiology with their properties, If this, however, 

 were the whole matter, the problems of anatomy and physi- 

 ology would be much simpler than they actually are. The 

 knowledge about the living Body obtained by studying only 

 the forms and functions of the individual tissues would be com- 

 parable to that attained about a great factory by studying 

 separately the boilers, pistons, levers, wheels, etc., found in 

 it, and leaving 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 per- 

 form very different duties, dependent upon the way in which 

 it is connected with other parts, so in the Body any one tissue, 

 although its essential properties are everywhere the same, 

 may by its activity subserve very various uses according to 

 the manner in which it is combined with others. For ex- 

 ample: 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 ex- 

 actly similar nerve-fibre running from the brain to the mus- 

 cles will, also by virtue of its conductivity, when its ending 

 in the brain is excited by a change in a nerve-cell connected 

 with it, stir up the muscle to contract 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 on differences in the properties of the nerve-fibres them- 

 selves. 



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 ordi- 

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

 complex organ, containing not only contractile muscular tis- 

 sue, but supporting and uniting connective tissue and con- 

 ductive nerve-fibres, and in addition a complex commissariat 

 arrangement, composed in its turn of several tissues, con- 

 cerned in the food-supply and waste removal of the whole 

 muscle. The anatomical study of a muscle has to take into 



