TISSUES. 39 



which was stated the result of yet deeper analysis — that all 

 organisms have a cellular structure and origin. The 

 simplest animals (Protozoa) are typically single cells or unit 

 masses of living matter ; as such all animals begin ; but all, 

 except the simplest, consist of hundreds of these cells united 

 into more or less homogeneous companies (tissues), which 

 may be compacted, as we have seen, into organs. If we 

 think of the organism as a great city of cells, the tissues 

 represent streets (like some of those in Leipzig), in each of 

 which some one kind of function or industry predominates. 



Since Leydig gave a strong foundation to comparative 

 histology in his remarkable " Lehrbuch der Histologic des 

 Menschen und der Thiere" (Frankfurt, 1857), the study has 

 been prosecuted with great energy, and has been constantly 

 stimulated by improvements in microscopic apparatus and 

 technique. 



The student should read the introductory chapters in one 

 of the numerous works on histology, so as to gain a general 

 idea of the characters of the different tissues. 



There are four great kinds, — epithelial, connective, 

 muscular, and nervous. 



(a) Epithelial tissue is illustrated by the external layer of the skin 

 (epidermis), the internal (endothelial) lining of the food canal and its 

 outgrowths, the lining of the body-cavity, etc. ; by the early arrange- 

 ments of cells in all embryos ; and by the simplest Metazoa, such as 

 Hydra, whose tubular body is lined by two layers of epithelium. 

 Embryologically and historically, epithelium is the most primitive kind 

 of tissue. It may be single layered or stratified ; its cells may be 

 columnar, scale-like, or otherwise. The cells may be close together, 

 or separated by intercellular spaces, and they are often connected by 

 bridges of living matter. Nor are the functions of epithelium less 

 diverse than its forms, for it may be ciliated (effecting locomotion, 

 food-wafting, etc.), or sensitive (and as such forming sense organs), or 

 glandular (liberating certain products or even the whole contents of its 

 cells), or pigmented (and thus associated with respiration, excretion, 

 and protection), or covered externally with a sweated-off cuticle, 

 susceptible of many modifications (especially of protective value). 



(6) Connective tissue. — This term is somewhat like the title ' ' worms. '' 

 It includes too many different kinds of things to mean much. It 

 represents a sort of histological lumber-room. 



The embryologists help us a little, for they have shown that almost 

 all forms of connective tissue are derived from the mesoderm or middle 

 layer of the embryo. As this mesoderm usually arises in the form of 

 outgrowths from the gut, or from ("mesenchyme") cells liberated at 

 an early stage from either (?) of the two other layers of the embryo 



