334 DUCTLESS GLANDS. 



pericardium, which on account of its fibrous layer is termed fibro- 

 serous ; (4) tunica vaginalis testis. 



CIRCULATING LYMPH. 



The lymph and its source having been discussed (p. 302), need 

 not again be referred to. It is taken up by the lymphatic capil- 

 laries in the tissues by endosmosis, and, as it accumulates there, 

 gradually fills the larger vessels, and, as it is readily discharged 

 into the venous system, there is set up a current which constitutes 

 the lymphatic circulation. It is to be noted, however, that there 

 is no true circulation, as in the case of the blood. The blood goes 

 out from the heart and returns again, completing a circuit, but 

 here the fiow is always in one direction, toward the heart. 



Additional aids to the endosmotic force in producing the move- 

 ment of the lymph are the contractions of the muscles of the body, 

 by which, as in the veins, the lymphatics are compressed, and the 

 lymph, being prevented by the valves from flowing back, is pro- 

 pelled toward the heart. The pressure exerted by the walls of 

 the aorta in its pulsations compresses the thoracic duct in a similar 

 manner, and, as this possesses valves, the onflow of the lymph 

 and chyle is favored. The force of aspiration of the thorax is 



FIG. 179. Endothelial cells from small artery of the mesentery of a rabbit : stained 

 with silver nitrate and hematoxylin (Huber). 



also a factor in the movement of the lymph, acting as was stated 

 in the case of the venous blood. It has been estimated that the 

 amount of lymph absorbed daily in a human adult is about 2000 

 grams, and of lymph and chyle together 3000 grams. 



DUCTLESS GLANDS. 



This term includes the spleen, the thyroid and parathyroids, 

 the thymus, the suprarenal capsules, the pineal gland, the pituitary 

 body, the carotid and coccygeal glands, and the lymphatic glands, 

 the last of which we have already considered. They have re- 

 ceived this name because they lack secretory ducts. For the 

 reason that they are believed by some to have important relations 

 to the blood they are sometimes described as blood-glands or vas- 

 cular glands. 



We have seen that although the lymphatic glands have no 



