SECRETING GLANDS. 319 



cells. Glands are provided also with lymphatic vessels and 

 nerves. The distribution of the former is not peculiar, and 

 need not be here considered. Nerve-fibres are distributed 

 both to the bloodvessels of the gland and to its ducts ; and, 

 in some glands, it is said, to the secreting cells also. 



The structure of the elementary portions of a secreting ap- 

 paratus, namely, epithelium, simple membrane, and blood- 

 vessels, having been already described in this and previous 

 chapters, we may proceed to consider the manner in which 

 they are arranged to form the varieties of secreting glands. 



SECRETING GLANDS. 



The secreting glands are the organs to which the office of 

 secreting is more especially ascribed : for they appear to be 

 occupied with it alone. They present, amid manifold diversi- 

 ties of form and composition, a general plan of structure, by 

 which they are distinguished from all other textures of the 

 body; especially, all contain, and appear constructed with 

 particular regard to, the arrangement of the cells, which as 

 already expressed, both line their tubes or cavities as an epi- 

 thelium, and elaborate, as secreting cells, the substances to be 

 discharged from them. 



For convenience of description, they may be divided into 

 three principal groups, the characters of each of which are de- 

 termined by the different modes in which the sacculi or tubes 

 containing the secreting cells are grouped : 



1. The simple tubule or tubular gland (A, Fig. 105), exam- 

 ples of which are furnished by the several tubular follicles in 

 mucous membranes : especially by the follicles of Lieberkiihn 

 in the mucous membrane of the intestinal canal (p. 241), and 

 the tubular or gastric glands of the stomach (p. 217). These 

 appear to be simple tubular depressions of the mucous mem- 

 brane on which they open, each consisting of an elongated 

 gland- vesicle, the wall of which is formed of primary mem- 

 brane, and is lined with secreting cells arranged as an epithe- 

 lium. To the same class may be referred the elongated and 

 tortuous sudoriparous glands of the skin (p. 338), and the 

 Meibomian follicles beneath the palpebral conjunctiva ; though 

 the latter are made more complex by the presence of small 

 pouches along their sides (B, Fig. 105), and form a connecting 

 link between the members of this division and the next, as the 

 former by their length and tortuosity do between the first di- 

 vision and the third (D, Fig. 105). 



2. The aggregated glands, including those that used to be 

 called conglomerate, in which a number of vesicles or acini are 



