TISSUES, ORGANS, AND SYSTEMS. 105 



follicles of those included under the fourth form, while the 

 remaining parenchyma of the spleen possesses peculiar cor- 

 puscles, not yet completely investigated, which seem to indicate 

 an energetic, retrogressive 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 pituitary body, it 

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

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

 stances 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, whilst in the intestinal follicles, it is doubtful 

 whether they excrete substances into the intestine, or receive 

 them from thence to give them up again to the vessels. In 

 the lymphatic glands, the ducts supply the glandular follicles 

 with matters which they take up again when further elaborated. 



The development of the blood-vascular glands is still very 

 obscure ; although this much appears certain, that most of them 

 are developed without the participation of the intestinal epithe- 

 lium, either from the fibrous wall of the intestine or from the 

 same blastema as that which produces the sexual glands. The 

 thymus and thyroid alone are to be regarded, according to 

 Remak, as diverticula of the intestinal canal. 



The nutrition of most of these glandular structures is very 

 energetic, as the abundance of the blood they contain and their 

 frequent morbid alterations show : the hypophysis cerebri and 

 the supra-renal capsules alone, in this respect, occupy a lower 

 grade. 



Literature. — A. Ecker, art. ' Blood- vascular Glands,' in 

 'Wagner's Handw. d. Phys.', Bd. IV, 1849. [H. Gray, ' On the 

 Development of the Ductless Glands in the Chick/ Philosoph. 

 Trans., 1852. — Eds.] 



