CHAPTEE XVI. 



THE LYMPHATIC SYSTEM. 



IN addition to the connected series of canals by which the blood 

 passes in a continuous round through the arteries, capillaries, and veins, 

 there is also a system of vessels, leading only from the periphery toward 

 the centre, and discharging into the great veins near the heart the fluids 

 which have been absorbed in the solid tissues of the body. The fluid 

 contained in these vessels is nearly or quite colorless, especially in thin 

 layers, and from its transparent and watery appearance is called the 

 " lymph," and the vessels themselves constitute what is known as the 

 lymphatic system. 



As the blood circulates through the capillaries under the influence of 

 the arterial pressure, certain of its ingredients transude through the vas- 

 cular walls and penetrate the interstices of the anatomical elements of 

 the tissues. An increased pressure upon the blood, either from arterial 

 congestion or from obstruction to the venous current, will increase the 

 amount of transudation, producing an cedematous condition of the part, 

 which is first perceptible in the loose connective tissue, but which may 

 afterward involve the more compact substance of the organs. In the 

 normal state of the circulation, this interstitial fluid, which is the real 

 source of nutrition for the solid parts, does not, however, stagnate in 

 contact with them, but is renewed by a continual change. As fresh sup- 

 plies need to be drawn from the circulating blood, the older portions are 

 removed by absorption and returned to the centre of the circulation by 

 the lymphatic vessels. Thus these vessels may be considered as com- 

 plementary in their function to the veins. The blood, containing the red 

 globules, requires to be rapidly and abundantly returned to the lungs by 

 the veins, in order to regain the oxygen necessary for its continued vital- 

 ity ; while the lymphatics collect more gradually the fluids which have 

 served for the slower process of nutrition and growth. 



Anatomical Structure and Arrangement of the Lymphatic System. 

 In structure the lymphatics do not essentially differ from the blood- 

 vessels, their principal peculiarity being that their walls are more delicate 

 and transparent. This circumstance, together with the colorless nature 

 of their contents, renders them less easily recognizable by dissection. 

 Those of larger and medium size consist of three coats, similar, in 

 general characters, to the corresponding tunics of the bloodvessels. 

 According to the observations of Kolliker, the external coat alone is 

 distinguished from that of the veins by the possession of smooth mus- 

 (354) 



