There is a certain set of organs which have but slight traits of resemblance to one another, and in consequence of the want of more accurate knowledge as to their exact function, and the fact that they do not pour their products into ducts, but probably into the blood current, are commonly grouped together as ductless or blood glands.
It has been shown that a great part of the absorbed nutrient material passes through a special set of vessels called the lacteals or lymphatics, and in so doing has to traverse peculiar organs called lymphatic glands, where it is no doubt modified, and has added to it a number of cells (lymph corpuscles) which subsequently are poured into the large veins with the lymph and become important constituents of the blood.
Fig. 156. Vertical section of the Supra-renal Capsule. (Eberth).
1. Cortex. 2. Medulla, a. Fibrous capsule. b. External cell masses. c. Columnal layer, d. Internal cell masses, e. Medullary substance, in which lies a large vein, partly seen in section,f.
Some of these blood glands are doubtless nearly akin to the lymphatic glands already described (Fig. 151), their duty being the further elaboration and perfection of the blood. In this group are commonly placed the supra-renal capsules, the thyroid, the thymus, and the spleen.
Fig. 157. Section of the Thyroid Gland of a child, showing two complete sacs and portions of others. The homogeneous colloid substance is represented as occupying the central part of the cavity of the vesicles, which are lined by even cubical epithelium. (Sc/iafer).