3oo 



HISTOLOGY 



sec. ^ 



sec. fi.-- 



Pancreatic tissue. The pancreas of a frog is a compound tubular 

 gland. The tubules are incased by a delicate membrana propria, 

 and are held together by a connective tissue, 

 which carries the nerve and vascular supply. 

 The wall of the tube is formed by simple 

 columnar epithelium. The cytoplasm of a 

 pancreatic cell bears a variable number of 

 fibrils, some of which are always found in 

 the basal third of the cell. The distal end 

 of the cell is usually granular and filled 

 with spherical secretion particles. The nu- 

 cleus always lies in the cytoplasm bearing 

 these fibrils. There is frequently found in 



FIG. 270. Pancreas cell from sala- . 11 



mander. sec., secretion substance; a pancreatic cell a round, dense body lying 



^..secretion fibrils. (After MATH- near tne nuc leus, called the paranucleus or 



nebenkern. This has been interpreted by 



Mathews as a tangle or knot of the fibrils that are commonly found in 

 the pancreatic cells (Fig. 270). 



Gastric tissue. The gastric glands of a muskrat are tubular. 

 The glands are supported in the mucosa of the stomach. These glands 

 open through depressions or crypts into the stomach. Each gland is 

 incased in a membrana propria. The glandular epithelium is composed 

 of two types of cells. The smaller and more numerous ones are small 

 columnar cells. The cytoplasm is rather dense ; the nucleus is round and 

 located near the center of the cell. They much resemble serous cells. 

 They receive the name of chief cells (Fig. 271, A, c.c.). These cells 

 elaborate a digestive fluid which is active in an acid medium. Crowded 

 back by the chief cells and lying more remote from the lumen of the tube 

 are larger cells which secrete hydrochloric acid. These are known as 

 the acid cells. An acid cell is usually rather large. Its original form 

 was columnar, but in most cases the acid cell has a shape conforming 

 to its position. The cytoplasm is reticular in appearance or highly 

 vacuolated, and is not so dense as the cytoplasm of a chief cell or a serous 

 cell. The nucleus is spherical and centrally placed. Although lying 

 remote from the lumen, there is always a passage between the chief cells 

 for the secretions of the acid cells to pass out (Fig. 271, A, a.c.}. In 

 the birds these two types of cells form different glands, so that there are 

 glands lined with chief cells and others with acid cells. These glands lie 

 in different parts of the enteron. The acid glands are always anterior 

 to the glands with the ferment-secreting cells. The latter are tubular 

 glands that dip into the submucosa of the gizzard (see Fig. 251). 

 The acid glands are found in the proventriculus. Each gland is a com- 

 pound gland. There is a central tube which is lined with an epithelium 



