CHAPTER VII. 

 THE LYMPHATIC SYSTEM. 



IX addition to the series of connected canals, through which the 

 blood passes in a continuous round by the arteries, capillaries, and 

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

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

 the materials which have been absorbed from the tissues. The fluid 

 in these vessels is nearly colorless, and from its transparent and 

 watery appearance is called the "lymph, 1 ' the vessels themselves 

 constituting the lymphatic system. 



As the blood moves through the capillaries under the influence of 

 the arterial pressure, certain of its ingredients transude through the 

 vascular walls and penetrate the interstices of the tissues. An in- 

 creased pressure of the blood, either from arterial congestion or from 

 obstruction to the venous current, will increase the amount of transu- 

 dation, producing an cedernatous condition, 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 circula- 

 tion, this interstitial fluid, which is the source of nutriment for the 

 solid parts, is renewed by continual change. As fresh supplies are 

 drawn from the circulating blood, the older portions are removed by 

 absorption and returned to the centre of the circulation by the lym- 

 phatic vessels. Thus these vessels may be considered as complementary 

 in function to the veins. The blood, containing the red globules, is 

 rapidly returned to the lungs by the veins, to regain the necessary 

 oxygen ; while the lymphatic vessels collect more gradually the fluids 

 which have served for nutrition and growth. 



General Structure and Arrangement of the Lymphatic System. 

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

 vessels, their main peculiarity being the greater delicacy and trans- 

 parency of their walls. Those of larger and medium size consist of 

 three coats, similar in general character to the corresponding tunics 

 of the blood-vessels. According to Kolliker, the external coat alone 

 is distinguished from that of the veins by the presence of muscular 

 fibres arranged in a longitudinal and oblique direction ; as seen in 

 lymphatics of 0.2 millimetre in diameter and upward. Like the veins, 

 they are provided with numerous valves, opening toward the heart 

 and closing toward the periphery. The smallest lymphatic vessel- 

 have only a single coat, composed of flattened, epithelium-like, nucleated 



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