These bodies may be likened to oval or rounded masses of sponge, into which the lacteals convey chyle and the lymphatics lymph.
Fig. 183 represents the structure of one of these glands. The gland has an investing coat or capsule (7) which completely surrounds it. From the capsule fibrous strands (6) pass into the gland, dividing it off like partitions into spaces. The spaces round the circumference (or cortex) of the gland are of considerable size, and are more or less oval (3), while the spaces towards the centre (or medulla) are irregular in shape, and smaller. The spaces are almost completely filled with masses of material, consisting of a net-work of very delicate connective tissue, in which white cells (4) of various sizes are entangled. This sort of tissue is called " adenoid", or gland tissue, from the Greek, adeen, a gland. But the masses of tissue do not quite fill the spaces. Between the outer surface of the mass and the wall of the space are channels (5), and the channel round one mass communicates with that of another, and those round the edge communicate with those in the centre, so that the gland might be looked upon as a mass of gland tissue broken up into numerous little clumps by a series of irregularly winding and communicating channels. The channels, moreover, are not perfect fair-ways. They are crossed and recrossed by spans of the delicate tissue of the gland, so that the whole structure becomes not unlike that of a sponge. Now the lacteal vessels join the mesenteric gland at the margin or outside (l), and pour their fluid contents into the channels there. From them the fluid filters its way to the channels of the centre, bathing and penetrating the gland tissue in its course, and finally joins other vessels (2), by which it is carried away from the gland. Through these comparatively free channels the chyle or lymph makes its way, easily entering the gland by the afferent, and escaping by the efferent vessels, and it then carries away from the gland but few leucocytes. When, however, the pressure of the fluid entering the gland is augmented, either by the process of digestion and the contraction of the villi in the case of the lacteals, or by active muscular movements in the case of the lymphatics, the lymph then percolates the substance of the gland and carries off with it by the efferent vessels large numbers of white cells, which are swiftly poured into the blood, and are believed to constitute one of the sources from which the blood corpuscles are recruited.
The main agents in effecting the movement of the lymph along its vessels are: (l) The force of the heart, which drives the plasma through the walls of the blood-vessels into the tissues; (2) the muscular movements of the body, generally aided by the valves present in all lymphatic vessels and the minute muscles surrounding the lacteals in the villi. The pressure under which the lymph moves is very low, and it is discharged from the thoracic duct into that part of the blood vascular system where the pressure is lowest, namely, into the veins just before their entry into the right auricle of the heart.
Fig. 183. - Section of Lymphatic Gland (diagrammatic).
1 Afferent Lymph Vessel. 2 Efferent Lymph Vessel. 3 Cortical Substance. 4 Lymphatic Tissue. 5 Lymph Path. 6 Trabecular sent in from 7 Fibrous Capsule.
When the outward movement of the lymph towards this point is impeded it accumulates in the vessels beyond the obstruction. This condition is more marked, and appears earlier if the obstruction be of such a nature as to affect the venous system as well as the lymphatic. The conditions known as oedema and dropsy are then established.
The composition of the lymph is very similar to that of the plasma of the blood, but it contains more water and less proteid matter. In the case of the horse there are about ninety-five to ninety-six per cent of water, and four or five per cent of solids, of which about three parts are albuminous or proteid substances, the remainder consisting of salts, the most abundant of which is common salt or sodium chloride.
The composition of chyle, consisting chiefly of the digested materials contained in the alimentary canal, varies with the nature of the food and the period of digestion at which it is examined. In the fasting animal it does not differ materially from the lymph, but, with an oily diet like milk, the proportion of fat undergoes great increase, and the lacteals become conspicuous by their white colour. After passing through the lymphatic glands, the lymph and chyle alike acquire the power of coagulating or clotting, though in both instances the clot is feeble and soft.