426 THE POPULAR SCIUNCU MONTHLY. 



as a compound of so many tissues, each tissue corresponding to one 

 of the fundamental qualities of j^rotoplasm, to the development of 

 which it is specially devoted by the division of labor. It must, how- 

 ever, be remembered, that there is a distinct limit to the division of 

 labor. In each and every tissue, in addition to its leading quality, 

 there are more or less pronounced remnants of all the other proto- 

 plasmic qualities. Thus, though we may call one tissue joar excellence 

 metabolic, all the tissues are, to a greater or less extent, metabolic. 

 The energy of each, whatever be its particular mode, has its source 

 in the breaking up of the protoplasm. Chemical changes, including 

 the assumption of oxygen and the production, complete or partial, of 

 carbonic acid, and therefore also entailing a certain amount of se- 

 cretion and excretion, must take place in each and every tissue. And 

 so witli all the other fundamental properties of protoplasm ; even con- 

 tractility, which, for obvious mechanical reasons, is soonest reduced 

 where not wanted, is present in many other tissues besides muscle. 

 And it need hardly be said that each tissue retains the power of as- 

 similation. However thoroughly the material of food be prepared by 

 digestion and subsequent metabolic action, the last stages of its con- 

 version into living protoplasm are effected directly and alone by the 

 tissue of which it is about to form a part. 



Bearing this qualification in mind, we may draw up a physiologi- 

 cal classification of the body into the following fundamental tissues : 



1. The eminently contractile : the muscles. 



2. The eminently irritable and automatic : the nervous system. 



3. The eminently secretory or excretory : digestive, urinary, and 

 pulmonary, etc., epithelium. 



4. The eminently metabolic: fat-cells, hepatic cells, lymphatic and 

 ductless glands. 



5. The reproductive : ovary, testis. 



6. The indiffei'ent or mechanical : cartilage, bone, etc. 



All these separate tissues, with their individual characters, are, 

 however, but parts of one body ; and in order that they may be true 

 members working harmoniously for the good of the whole, and not 

 isolated masses, each serving its ow^n ends only, they need to be bound 

 together by coordinating bonds. Some means of communication must 

 necessarily exist between them. In the mobile, homogeneous body of 

 the amoeba, no special means of communication are required. Simple 

 diffusion is sufficient to make the material gained by one part common 

 to the whole mass, and the native protoplasm is physiologically con- 

 tinuous, so that an explosion set up at any one point is immediately 

 pi'opagated throughout the whole irritable substance. In the higher 

 animals the several tissues are separated by distances far too great 

 for the slow process of diffusion to serve as a sufficient means of com- 

 munication, and their primary physiological continuity is broken by 

 their being imbedded in masses of formed material, the product of 



