GLANDS. 85 



brane), which is usually very delicate and structure- 

 less ; sometimes it is more dense, being strengthened 

 externally by a layer of fibrillated tissue (as in the 

 testicles, lungs, etc.). Its internal surface is covered 

 by a simple layer of epithelial cells, generally many- 

 sided ; or, these are arranged in strata, so as to fill 

 completely the cavities of the secreting vesicles, or 

 tubules. Upon the external surface of their basement 

 membrane blood-vessels ramify, so as to form a capil- 

 lary web, or network, with meshes of varying size 

 in different glands. 



Nerves accompany the blood-vessels, but aTe rela- 

 tively less numerous ; their mode of termination is 

 not yet fairly known, but it is probably by means of 

 free extremities. 



The excretory canals, or ducts, of glands, have an Ducts, 

 external coat which is usually formed by the inter- 

 lacement of connective and elastic fibres, together 

 with fibres of unstriped muscle ; internally they are 

 lined by an epithelial layer, the cells of which diifer 

 from those of the parenchyma of the gland. The 

 ducts of some glands contain minute clusters of vesi- 

 cles in the thickness of their walls (liver, pancreas, 

 lung). 



SECT. I. GLANDS CONSISTING OF CLUSTERS OF VESI- Glands formed bj 



clust*8 of vesi- 



CLES. The glands of this variety scarcely differ from cles - 

 each other in structure, except in the minute details 

 which belong to the disposition and arrangement of 

 their epithelial element. Therefore, in order to 

 escape the useless repetitions which a separate de- 

 scription of each gland would of necessity involve, 

 we shall study the structure of certain individual 



