INTRODUCTORY. 5 



individual, certain units are set apart in the form of ovary and 

 testis. In these all the properties of protoplasm are distinctly 

 subordinated to the work of growth. 



Lastly, there are certain groups of units, certain tissues,. which 

 are of use to the body of which they form a part, not by reason of 

 their manifesting any of the fundamental qualities of protoplasm, 

 but on account of the physical and mechanical properties of certain 

 substances which their protoplasm has been able by virtue of its 

 metabolism to manufacture and to deposit. Such tissues are bone, 

 cartilage, connective tissue in large part, and the greater portion 

 of the skin. 



We may therefore consider the complex body of a higher 

 animal as a compound of so many tissues, each tissue correspond- 

 ing to one of the fundamental qualities of protoplasm, to the 

 development of which it is specially devoted by the division of 

 labour. It must however be remembered that there is probably 

 a distinct limit to the division of labour. In each and every 

 tissue, in addition to its leading quality, there are more or less 

 pronounced remnants or at least some traces of all the other 

 protoplasmic qualities. Thus, though we may call one tissue par 

 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 secretion and excretion, must take place in 

 each and every tissue. And so with all the other fundamental 

 properties of protoplasm ; even contractility, 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 assimilation. However 

 thoroughly the material of food be prepared by digestion and 

 subsequent metabolic action, the last stages of its conversion 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 physio- 

 logical classification of the body into the following fundamental 

 tissues : 



1. The eminently contractile : the muscles. 



2. irritable and automatic : the nervous system. 



3. secretory, or excretory : digestive, urinary, 



and pulmonary, &c., epithelium. 



4. metabolic : fat-cells, hepatic cells, lymphatic 



and ductless glands, &c. 



5. reproductive : ovary, testis. 



6. The indifferent or mechanical : cartilage, bone, &c. 



All these separate tissues, with their individual characters, are 



