EPITHELIAL TISSUE. 37 



III. TISSUES. 



Zoological anatomists, of whom Cuvier may be taken as a 

 type, analyse animals into their component organs, and dis- 

 cover the homologies between one animal and another. But 

 as early as 1801, Bichat had published his Anatomic Generate, 

 in which he carried the analysis further, showing that the 

 organs were composed of tissues, contractile, nervous, gland- 

 ular, &c. In 1838-9, Schwann and Schleiden formulated 

 the " cell theory," in 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 homo- 

 geneous 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 Histologie 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, mus- 

 cular, 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, &c. ; by the early arrangements 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 



