GENERAL ANATOMY. 87 



Classification of Tissues. Since in every tissue its 

 function interests us most, it would be natural to base the 

 classification of tissues upon the function and the intimate 

 structure connected therewith. 



For a long time the tissues have been arranged in 

 four groups: I. Epithelial tissue ; 2. Supporting tissue ; 3. 

 Muscle tissue; 4. Nervous tissue. Within these, how- 

 ever, certain constituent parts of the animal body find no 

 shelter, constituent parts to which indeed the term " tissue" 

 is scarcely applicable : these are the sexual cells, the blood, 

 and the lymph. The former may be spoken of in connec- 

 tion with the epithelium, the latter in connection with the 

 supporting substances. 



i. EPITHELIAL TISSUES. 



Morphology of Epithelial Tissues. On several 

 grounds the epithelia must be first considered. They 

 are the oldest tissues ; they are the first to appear in the 

 animal kingdom, there being animals which consist only of 

 epithelia. Further, each separate organism during the first 

 stages of embryonic life consists only of epithelial layers, 

 the germ-layers. With this is also connected the fact that 

 in epithelial tissues the cells have undergone the least 

 degree of histological change, and that the formation of 

 plasmic products is subordinated. 



Function of Epithelium. The most important purpose 

 of the epithelium is to form over surfaces a protecting and 

 excluding covering, equally valuable whether the surfaces 

 are external (surface of the body) or caused by cavities in 

 the interior of the body (the body cavity, lumen of the gut, 

 of the blood-vessels). The importance of the epithelia in 

 this respect is shown by the fact that if the protecting 

 layers are removed, inflammation arises and continues until 

 the epithelium is regenerated. Only exceptionally do areas 

 occur which are free from epithelium ; the teeth of verte- 

 brates, the antlers of stags, are parts of the body which, on 



