TISSUES, ORGANS, AND SYSTEMS. 



117 



closed follicles like those above mentioned, but, so far as v;e as yet 

 know, without vessels in their interior. 



Fis. 43. 



Fipr. 44. 



,/ 



d. Tlie lymphatic glands, which appear to consist of follicles like 

 those of the Peyerian patches. 



4. As a cellulai' parenchyma, which contains numerous closed follicles 

 like those just described : Spleen. 



5. As racemose, aggregated, glandular vesicles opening into a common 

 closed canal or broad space, whose thick walls are formed of a delicate 

 investment of connective tissue, and of a soft substance consisting of 

 many nuclei and of vessels : Thymus. 



We know little of the chemical nature of these organs, which are all 

 more or less richly supplied Avith bloodvessels. Those enumerated 

 under 1, 2, 3, and 5, contain much protein and fat in their tissue, as 

 also do the follicles of those included under the fourth form, while the 

 remaining parenchyma of the spleen possesses peculiar corpuscles, not 

 yet completely investigated, which seem to indicate an energetic, retro- 

 gressive metamorphosis. We know little of the physiological functions 

 of these glands ; and here it need merely be remarked, that in the 

 spleen, the thyroid, the thymus, the supra-renal capsules, and the pitui- 

 tary body, it can only be the blood which yields material to them, and 

 only the blood- and lymph-vessels which again receive the substances 

 given off externally or internally (thymus) by them. In the follicular 

 glands of the mouth and pharynx, the secretions are poured into the 

 wider cavities of the glands, and ultimately into those organs, Avhilst in 



Fig. 43. — A few of the glandular vesicles from the thyroid gland of a child: a, connective 

 tissue between them; 6, membrane of the glandular vesicles; c, their epithelium — Magni- 

 fied 250 diameters. 



Fig. 44. — A Malpighian corpuscle from the spleen of the ox : a, wall of the corpuscle ; b, 

 contents ; c, wall of the artery upon which it is seated ; (/, sheath of the latter. — Magnified 

 150 diameters. 



