4 THE FUNDAMENTAL TISSUES. 



impossibility. The accumulation of units would be a hindrance to 

 welfare rather than a help. Hence, in the evolution of living 

 beings through past times, it has come about that in the higher 

 animals (and plants) certain groups of the constituent amcebiform 

 units or cells have, in company with a change in structure, been 

 set apart for the manifestation of certain only of the fundamental 

 properties of protoplasm, to the exclusion or at least to the 

 complete subordination of the other properties. 



These groups of cells, thus distinguished from each other at 

 once by the differentiation of structure and by the more or less 

 marked exclusiveness of function, receive the name of 'tissues.' 

 Thus the units of one class are characterized by the exaltation of 

 the contractility of their protoplasm, their automatism, metabolism 

 and reproduction 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 complete 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 

 further 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 

 epithelium 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 

 chemical 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 materials from the blood 

 and return their products back to the blood. They may be called 

 the metabolic tissues par excellence. 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 concerned), and probably many 

 other cellular elements in various regions of the body. 



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 



