EPITHELIAL AND ENDOTHELIAL TISSUE. 329 



is evidently due to the contraction of the living net-work of 

 the epithelium, on the one hand, and a rupture in the wall of 

 the epithelium, on the other. This aperture probably closes 

 immediately upon cessation of the contraction and reestablish- 

 ment of relative rest in the reticulum. Watery secretion is 

 accomplished by the lachrymal and the sweat glands. The latter 

 produce a fluid, varying greatly in the amount of its solid constit- 

 uents and its consistency at different times, which indicates that 

 the living matter of the epithelium itself has an influence on the 

 chemical nature of the secretion. At the approach of death, the 

 perspiration is inspissated and almost mucous in character. The 

 main function of the uriniferous tubules is the inspissation of 

 the fluid pressed out from the blood-vessels of the tufts of the 

 kidneys. 



Cb) The mucous secretion can be directly observed under the 

 microscope best on a minute particle, cut from the inner surface 

 of the small intestine of a frog, with the addition of a very dilute 

 solution of chromic acid or bichromate of potash ; pure water 

 acts with too much rapidity. We first observe a swelling of the 

 body near the outer or free surface of the columnar epithelium. 

 This can be accounted for by assuming that the epithelium 

 imbibes the liquid which causes the swelling of the meshes 

 of the bioplasson reticulum, and at the same time produces 

 a considerable stretching of the reticulum. A contraction 

 of the posterior portion of the bioplasson reticulum results^ 

 and this also assists in the enlargement of the anterior por- 

 tion. The covering cement-substance of the free surface pro- 

 trudes, and its delicate rods fall off. The cement-substance, after 

 having reached its utmost capacity of expansion, bursts, and a 

 pale, globular body is thrown out **. e., the swelled portion of 

 the epithelium, in which no trace of the former structure can be 

 seen. We must admit, however, the presence of a stretched 

 reticulum in the mucous globule, too, and also an extremely thin, 

 expanded, investing layer of the bioplasson. A number of the 

 above described pale mucous globules coalesce and form the jelly- 

 like mass called mucus. 



When the process of swelling goes on more slowly, the whole 

 bioplasson enlarges within its envelope of cement-substance, and 

 after being freed, the nucleated plastid, now termed " mucous 

 corpuscle/' still exhibits the net-like structure ; or we may see 

 isolated, torn granules in an active, so-called " molecular " mo- 

 tion, the broken particles of the former bioplasson reticulum. 



