THE DIFFERENTIATION OF TISSUES. 39 



auditory apparatus, and so on, will readily be thought of. 

 The study of the working of such complicated mechanisms 

 forms a very important part of physiology. 



Anatomical Systems. From the anatomical side a whole 

 collection of bodily organs agreeing in structure with on,e 

 another is often spoken of as a system ; all the muscles, for 

 example, are grouped together as the muscular system, and 

 all the bones as the osseous system, and so on, without any 

 reference to the different uses of different muscles or bones. 

 The term system is, however, often used as equivalent to 

 "apparatus": one reads indifferently of the "circulatory sys- 

 tem " or the " circulatory apparatus." It is better, however, 

 to reserve the term system for a collection of organs classed 

 together on account of similarity of structure; and "appa- 

 ratus " for a collection of organs considered together on ac- 

 count of their co-operation to execute one function. The 

 former term will then have an anatomical, the latter a phy- 

 siological, significance. 



The Body as a Working Whole. Finally it must all 

 through be borne in mind that not even the most complex 

 system or apparatus can be considered altogether alone as an 

 independently living part. All are united to make one living 

 Body, in which there is throughout a mutual interdepend- 

 ence, so that the whole forms one human being, in whom the 

 circulatory, respiratory, digestive, sensory and other appara- 

 tuses are constantly influencing one another, each modifying 

 the activities of the rest. This interaction is mainly brought 

 about through the conductive and co-ordinating tissues of 

 the nervous system, w r hich place all parts of the Body in com- 

 munication. But in addition to this another bond of union 

 is formed by the blood, which by the circulatory apparatus is 

 carried from tissue to tissue and organ to organ and so, bring- 

 ing materials derived in one region to distant parts, enables 

 each organ to influence all the rest for good or ill. 



Besides the blood another liquid, called lymph, exists in 

 the Body. It is contained in vessels distinct from those 

 which carry the blood, but emptying into the blood-vessels at 

 certain points. This liquid being also in constant movement 

 forms another agency by which products are carried from 

 part to part, and the welfare or ill-fare of one member en- 

 abled to influence all. 



