THE IMPORT OF PROTOPLASM. 425 



tility of their protoplasm, their automatism, metabolism, and repro- 

 duction, being kept in marked abeyance. These units constitute the 

 so-called muscular tissue. Of another tissue viz., the nervous the 

 marked features are irritability and automatism, with an almost com- 

 plete absence of contractility and a great restriction of the other 

 qualities. In a third group of units, the activity of the protoplasm is 

 largely confined to the chemical changes of secretion, contractility 

 and automatism (as manifested by movement) being either absent or 

 existing to a very slight degree. Such a secreting tissue, consisting 

 of epithelium-cells, forms the basis of the mucous membrane of the 

 alimentary canal. In the kidney, the substances secreted by the cells, 

 being of no farther use, are at once ejected from the body. Hence 

 the renal tissue may be spoken of as excretory. In the epithelium- 

 cells of the lungs, the protoplasm plays an altogether subordinate 

 part in the assumption of oxygen and the excretion of carbonic acid. 

 Still, we may, perhaps, be permitted to speak of the pulmonary epi- 

 thelium as a respiratory tissue. 



In addition to these distinctly secretory or excretory tissues, there 

 exist groups of cells specially reserved for the carrying on of chemi- 

 cal changes, the products of which are neither cast out of the body 

 nor collected in cavities for digestive or other uses. The work of 

 these cells seems to be of an intermediate character : they are engaged 

 either in elaborating the material of food that it may be the more 

 easily assimilated, or in preparing used-up material for final excretion. 

 They receive their material from the blood, and return their products 

 back to the blood. They may be called the metabolic tissues joar ex- 

 cellence. Such are the fat-cells of adipose tissue, the hepatic cells (as 

 far as the work of the liver other than the secretion of bile is con- 

 cerned), and, in general, the blood. 



Each of the various units retains, to a greater or less degree, the 

 power of reproducing itself, and the tissues generally are capable of 

 regeneration in kind. But neither units nor tissues can reproduce 

 other parts of the organism than themselves, much less the entire 

 organism. For the reproduction of the complex 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 in 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 sub- 

 stances which their protoplasm has been able, by virtue of its metab- 

 olism, to manufacture and to deposit. Such tissues are bone, carti- 

 lage, connective tissue in large part, and the greater portion of the 

 skin. 



We may, therefore, consider the complex body of a higher animal 



