TISSUES. 41 



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 undder 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). 



(b) Connective tissue. This term 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 

 (ectoderm or endoderm), we may say that connective tissue is primarily 

 derived from epithelium. 



The general function of ls connective tissue " is to enswathe, to bind, 

 and to support, but the forms assumed are very various. 



(a) The cells may be close together, without any intercellular 

 ' mortar '' or matrix. They may contain large vacuoles, and thus 

 produce the appearance of a network, or they may be laden with fat 

 or with pigment. 



(//) In other cases the cells of the connective tissue lie in a matrix, 



