64 GENERAL PRINCIPLES OF ZOOLOGY 



all the vital functions, these are incomplete because they mutually hinder 

 each other in their free development; the plasmic product, on the other 

 hand, has only the single function peculiar to it and can therefore per- 

 form this with greater comjileteness. Muscle iiljrilhe, the characteristic 

 elements formed by the muscle cells, have preserved of the various prop- 

 erties of protoplasm only the capability of contraction; but this contrac- 

 tion is much more energetic and stronger than the mere mo\'ement of 

 protoplasm. Nerve librilkr only transmit stimuli, but in far more rapid 

 and orderly manner than docs simple protoplasm. 



Classification of Tissues. — Since in every tissue its function interests 

 us most, it is natural to classify tissues by the function and the intimate 

 structure connected therewith. The tissues are arranged in four groups: 

 I. Epithelial tissue; 2. Supporting tissue; 3. INfuscular tissue; 4. Nervous 

 tissue. Within these, however, certain parts of the animal body, to which 

 indeed the term 'tissue' is scarcely applicable, find no shelter; these are the 

 sexual cells, the blood, and the lymph. The first may be spoken of in 

 connection with the epithelium, the others with the supporting substances. 



I. Epithelial Tissues. 



Morphology of Epithelial Tissues. — An epithelium is a layer of 

 cells covering any free surface, e-xternal or internal, of the body. The 

 epithelia must be considered first, because they are the oldest tissues; 

 the first to appear in the animal kingdom, there being animals which 

 consist only of epithelia. Further, every metazoan, during the first 

 stages of embryonic life, consists only of epithelial layers, the germ- 

 layers. With this is connected the fact that epithelial cells have under- 

 gone the least degree of histological change, and tliat tlie formation of 

 plasmic products is sul)ordinated. 



Function of Epithelium. — Epithelium forms a protecting and ex- 

 cluding covering over surfaces, equally valualile whether the surfaces are 

 external (surface of the body) or in cavities in the interior of the body 

 (the body cavity, lumen of the gut and blood-vessels). The importance 

 of the epithelia in this respect is shown liy the fact that if the protecting 

 layer be removed, inflammation arises and continues until the epithelium 

 is regenerated. Only exceptionally do areas occur which are free from 

 epithelium; the teeth of vertebrates, the antlers of stags, on account of 

 their hardness, can exist, at least for a time, without epithelial co^•ering. 



Glandular and Sensory Epithelia. — By their position epithelia are 

 suited for two other functions: all substances which ought to be removed 

 from the body — some because they have become useless, and consequently 



