54 



HISTOLOGY 



Compound saccular glands appear in the salivary glands of the mammals, 

 of which easily studied examples are to be seen in the lower part of the 

 tongue. 



Besides such compounding of the simple glands a second sort of 

 amplification may appear in which the walls of a primary acinus may 

 be invaginated into acini of a different character. These we shall 

 designate as the complex glands. A particularly good example of such 

 a complex gland is to be seen in the stomach region or proventriculus of 

 birds, of which the pigeon will furnish us with a good example (Fig. 57, G). 

 In this specimen a section will show that each of the numerous glands 

 opening into the lumen of the proventriculus is a simple saccular form 

 lined with an epithelium that is different from that of the surface upon 

 which it opens. The exact use of this primary gland epithelium is not 

 known, but judging from its homologies it should be pepsin producing. 

 Its staining reactions and general appearance, as well as the fact that 

 the chief ferment of digestion is produced elsewhere, would indicate that 

 this function has been lost. Opening from all sides into this primary 

 fundus are many smaller, secondary invaginations. These are lined with 

 a totally different kind of cell whose function has been found to be that 

 of producing hydrochloric acid. These cells are represented in Figure 

 268, B, in Chapter XV. 



Another complex gland is also a digestive gland whose primary and 

 secondary invaginations produce digestive ferment and hydrochloric 

 acid respectively. This is the tubular (usually) gland found in the 



stomach, and we shall examine the 

 form seen in the muskrat (see Fig. 

 268, A}. The point to be made 

 here is that in the mammal acid 

 gland we have a complex gland 

 whose secondary elements are uni- 

 cellular glands that have been 

 slightly retired from the primary 

 glands as secondary invaginations. 

 The ducts of glands form an in- 

 teresting study. The duct is usu- 

 ally an intercellular passage with 

 from three to six cells forming its 

 walls at any one transection. On 

 the one hand it may become much 

 larger when it carries off the secre- 

 tion products of very many gland bodies. In this case it is furnished 

 with strong connective tissue coverings or even muscle layers to 

 strengthen and contract it (see Fig. 58). On the other hand, the 



FIG. 58. Transverse section of part of a 

 medium-sized duct of the cat's submaxil- 

 lary gland. X 870. 



