Page:Popular Science Monthly Volume 11.djvu/441

Rh 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 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 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 material 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, 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 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