994 THE VASCULAE SYSTEM. 



system, the eye and the internal ear, are plentifully provided with lymph vessels, 

 which are divided, according to their positions, into superficial and deep vessels. 



The superficial lymph vessels lie in the skin and subcutaneous tissues; they 

 frequently accompany the superficial veins and, in the limbs, they join the deep 

 vessels in definitely localised situations. 



The deep lymph vessels drain the lymph from all parts of the body which lie 

 internal to the deep fascia; they tend to accompany the blood-vessels of the 

 various parts and organs. 



The lymph glands also are divided into superficial and deep groups. The 

 former lie in the superficial fascia and are comparatively few in number. They 

 are associated more particularly with the superficial lymph vessels of the limbs. 

 The deep lymph glands of the limbs are also comparatively few in number, but 

 those of the head, neck, and trunk are very numerous. 



Vasa Lymphatica. The smaller lymphatics are channels of various shapes 

 but of greater calibre than the blood capillaries. They anastomose freely together, 

 forming lymphatic plexuses. 



Their walls are formed by a single layer of endothelial cells of irregular shape 

 and sinuous outline, which are in close contact, externally, with the tissue elements 

 amidst which they lie. In some cases lymph vessels, or plexuses of lymph vessels, 

 surround the blood-vessels forming perivascular lymph channels. It has been 

 generally believed that such perivascular channels are numerous in the central 

 nervous system, but the researches of Bruce tend to show that, whilst the 

 lymphocytes travel along the walls of the blood-vessels of the central nervous 

 system, they pass along cleft-like spaces in the adventitial sheaths, and not in 

 distinct perivascular channels. 



As the lymph vessels attain a larger size their walls are strengthened by the 

 deposition of a layer of elastic fibres on the outer surface of the endothelial coat. 

 The fibres run longitudinally, and in some cases fuse together to form a fenestrated 

 elastic membrane. 



The walls of all the largest lymph vessels, like those of the blood-vessels, are 

 formed by three tunics, external, middle, and internal. The tunica intima consists 

 of a layer of endothelium covered externally by elastic fibres or fenestrated elastic 

 membrane. The tunica media is formed of transverse and oblique unstriped muscle 

 fibres, intermingled with elastic fibres. The tunica externa consists of longitudinal 

 connective tissue elements, with which are intermingled oblique and longitudinal 

 unstriped muscle fibres ; the latter feature being peculiarly characteristic, for it is 

 not met with in the blood vascular system, except in the walls of the large veins. 



Bicuspid valves, formed by semilunar folds of the tunica intima are extremely 

 numerous in the lymph vascular system. They are either absent or they are few 

 and incompletely developed in the small vessels and the lymph plexuses, but they 

 are very prevalent in the larger vessels ; consequently when the latter are injected 

 they assume a modulated outline. Valves are present also at the entrances of the 

 great lymph channels into the venous system. 



Lymphoglandulae. Lymph glands are globular, ovoid, flattened, or irregular 

 bodies, and each gland presents a localised depressed area which is known as the 

 hilum. The lymph glands vary considerably in size, some being no larger than a 

 pin's-head, whilst others are as large as a bean. In colour they are usually grayish 

 pink, but the tint varies with the position, vascularity, and state of activity of 

 the gland. The lymph glands of the lung are generally blackened by the deposi- 

 tion of carbonaceous material in their substance, and those of the liver and spleen 

 frequently have a brownish hue. The lymph glands of the mesentery are creamy 

 or white whilst the chyle is rapidly passing through them, but when the absorp- 

 tion of food-material from the intestine ceases they become a rosy pink. 



The lymph glands are embedded in the connective tissues, some lying super- 

 ficially in the subcutaneous tissues, but the majority more deeply and usually at 

 the sides of the great blood-vessels. As a rule, they are arranged in groups of from 

 two to fifteen, but a few of those which lie in the subcutaneous tissues are solitary. 

 They form centres to which afferent lymphatic vessels converge, and from 

 which efferent vessels pass onwards to other glands or to the larger lymph channels. 



