TISSUES, ORGANS, AND SYSTEMS. 



101 



Fig. 12. 



they contain in their interior peculiar substances, — as fat, the 

 constituents of the bile, of the urine, of the 

 gastric juice, mucus, &c, and thence assume 

 a specific character. 



The true glands either separate certain 

 constituents from the blood, or by means 

 of it, elaborate peculiar substances or struc- 

 tural elements, and according as they do 

 the one or the other, is the import of their 

 separate parts different. In the former 

 glands the cells play a more subordinate 

 part, and are at most of importance, so far 

 merely, as they impede the passage of this or 

 that constituent of the blood, and allow only 

 certain of them to pass (kidneys, lachrymal 

 glands, small sudoriparous glands, lungs) ; 

 whilst in others, the cells take a very important 

 share in the formation of the glandular fluid, 

 by producing within them the specific secre- 

 tion, which then either drains out of them 

 (liver, mucous glands, gastric glands, pros- 

 tate, Cowper's glands, salivary glands, pancreas), or becomes 

 free by the gradual dissolution and breaking-up of the cells 

 themselves (lacteal glands, fat-glands, testis, larger sudoriparous 

 and ceruminous glands). In the former case, as in the 

 Graafian follicles, a peculiar cell-development may take place 

 in the secretion which is formed, whilst in the latter, new 

 elements continually arise in place of those gland-cells which 

 are removed as they attain their full development, in con- 

 sequence of which the character of these cells as a coating 

 of the glandular canals is frequently lost, and they appear 

 simply as a part of the secretion {testis, lacteal gland during 

 lactation). All the glands here mentioned, with the ex- 

 ception of the sexual, are developed from the internal and 

 external epithelial structures of the body, conjoined with 

 the vascular membranes which support these epithelia. 

 Some of them originate as involutions of these membranes, 

 and retain the cavities throughout the course of their deve- 



Fig. 42. Gastric gland from the pylorus of the dog, with cylinder-epithelium : 

 a, larger glandular cavity; b, tubular appendages of it. 



