860 THE VASCULAR SYSTEM. 
tissue. The larger spaces are lined by a layer of flattened endothelial cells, with 
sinuous outlines, similar to the cells of the lymphatic capillaries; but the smaller 
spaces have no endothelal lning, and they are limited only by the cells of the 
tissue in which they lie. The precise nature of the communications between 
lymphatic spaces and lymph capillaries has not been definitely ascertained; but 
undoubtedly lymph passes from the spaces into the capillaries, and probably it 
does so because the spaces and vessels are directly continuous. 
As alternative explanations, 1t may be suggested either that the passage is due 
to the existence of stomata in the walls which. separate spaces from the capillaries, 
or that if may be due to transudation through intervening tissues. 
Lymph capillaries.—Lymph capillaries are not only much larger and more 
irregular than blood capillaries, but they are also larger than the lymphatic vessels 
into which they open. They are lined ‘by a single layer of endothelial cells which 
possess very sinwous outlines ; apparently they are in direct structural continuity 
with lymph spaces, and they anastomose freely together, forming plexiform 
labyrinths. } 
Lymphatic vessels.—The smallest lymphatic vessels are much narrower in 
calibre than the lymph capillaries with which they are continuous. Their walls 
consist of an internal lining of endothelial cells of fusiform shape and regular 
outline, and an outer layer of fine connective tissue. They are provided with 
numerous valves, and when distended have a beaded appearance. 
The larger lymph vessels possess three coats —(1) An internal coat (tunica 
interna ), formed by layer of endothelial cells, of fusiform shape, and of regular, but 
sinuous, outhne. (2) A middle coat (tunica media) of unstriped muscle fibres, 
arranged for the most part circularly, but some of which run obliquely or longi- 
tudinally. The interspaces between the muscle fibres are filled with a fine con- 
nective tissue. (3) An outer coat (tunica externa) of mixed white fibrous and 
elastic tissue, which is not sharply separated from the middle coat on the inner 
side or from the surrounding tissues on the outer side. It may also contain a few 
unstriped muscle fibres. In the largest vessels the two outer coats consist prin- 
cipally of muscle; they are, therefore, very friable. 
All the large vessels are provided with numerous bicuspid valves, which are 
formed by folds of the inner coat; and as the lumina of the vessels are enlarged 
just above the attachments of the valves, the vessels assume a characteristic 
beaded appearance when they are distended. 
Lymphatic vessels anastomose freely together, and the majority form communi- 
cating channels between different groups of lymphatic glands, leaving the more 
distal glands as efferent vessels and entering the more proximal glands as afferent 
vessels. Some of the lymphatic vessels, however, are afferent only ; ; they simply 
carry lymph from the periphery to the nearest glands; whilst others, which carry 
lymph from the last set of elands to the terminal trunks, are efferent only. 
The lymphatic vessels, unlike the veins which they usually accompany, do not 
increase greatly in calibre as they converge towards their terminations; they often 
branch, and they frequently anastomose together. In certain places, particularly 
the central nervous system and the spleen, lymph capillaries or even lymphatic 
vessels completely ensheath the smaller blood - vessels, forming perivascular 
lymphatic spaces. 
Lymphatic glands. — Lymphatic glands are globular, ovoid, flattened, or 
irreoular bodies, and each eland presents a localised de pressed area which is know n 
as the hilus. They 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 erayish pink, 
but the tint varies with the position, vascularity, and state of activity ‘of the oland. 
The glands of the lung are generally blackened by the deposition of carbonaceous 
material in their substance, and those of the liver and spleen have frequently a 
brownish hue. The glands of the mesentery are creamy or white whilst the chyle 
is rapidly passing through them, but when the absorption of food-material from 
the intestine ceases they become a rosy pink, 
Structure of Lymphatic Glands.—Lymphatic glands consist of (1) a skeleton or framework, 
(2) lymph sinuses, and (3) lymph follicles and cords. 
