EPITHELIAL AND ENDOTHELIAL TISSUE. 327 



ment membrane" of Bowman. Between the epithelia and the 

 hyaline layer a flat, delicate endothelial formation is often found, 

 which is well marked in the seminiferous tubules (Mihalkovits) 

 and in the uriniferous tubules (C. Ludwig). The hyaline layer of 

 the salivary glands exhibits, according to Boll, delicate radiating 

 spokes, emanating from the flat, central nucleus, which may be 

 considered either stellate connective-tissue corpuscles, or rib-like 

 ledges of the apparently structureless membrane. Outside of 

 this layer is a delicate interstitial structure of fibrous connective 

 tissue, freely supplied with blood-vessels and nerves. Connective 

 tissue envelops the larger glands, forming a dense, firm capsule, 

 called the tunica albuginea. 



The nerves of the interstitial connective tissue are of both the 

 medullated and non-medullated varieties, and not infrequently 

 small ganglionic elements are met with along their course. The 

 ultimate terminations of nerves are not yet fully understood; 

 but, as far as my own observations go, I feel inclined to regard 

 the termination represented in Fig. 140 as the most common. 



Very little is known about the lymphatics of the glands. The 

 so-called lymph-spaces, produced by parenchymatous injection, 

 are unquestionably artificial formations, and are not entitled to 

 the name of lymphatics. A complete system of lymph-vessels 

 is known to exist in only the testes. 



Performances of Epithelium. The principal function of epi- 

 thelium, besides the protection of the body, the transmission of 

 terminal nerve-fibers therefore of sensation, etc., is the elimina- 

 tion of effete material from the body, viz. : by secretion and excre- 

 tion. The horny, epidermal layer of the skin, on account of its 

 indifference, within certain limits of course, to the action of acids 

 and alkalies, and on account of its being a bad conductor of heat, 

 serves for a protection to the body and all cavities exposed to the 

 outer world. The chief office performed ly epithelia is unquestion- 

 ably that of secretion, which is carried on exclusively by epithelia, 

 arranged as glands. Every glandular formation is epithelial, and 

 each epithelial body may be looked upon as a gland, inasmuch as 

 all secretory action is based upon the function of each single 

 epithelium. All those processes which are considered the most 

 disgusting in the animal economy are performed by epithelia j 

 but it is epithelium, also, which produces the sweet, nourishing 

 liquid called milk, and all its derivations, and which gives rise to 

 the most essential material of generation viz. : the spermatozoids 

 and the ova. 



