The inferiority of the chyliferous system of birds to that of quadrupeds is seen even in the properties of the chyle, which is still, as in the lower tribes of vertebrata, a thin, colourless, and limpid fluid. The lacteal ves sels are now, however, more obvious, and more regular in their distribution, and are spread in more crowded layers above the mu cous and above the muscular coats of the in testine. They collect from the intestine and form numerous anastomosing plexuses on the mesentery, in place of the conglobate glands of mammalia and then proceed, with the lym phatics, to the receptaculum, which sends for ward two thoracic ducts to terminate, on each side of the neck, at the junction of the sub clavian with the jugular veins. (See AvEs.) The coats of the lacteals are still very thin and distensible in birds; their valves, which are more abundant on the trunks and branches than in reptiles, are still so incomplete as to allow injections to pass easily against their course, and although conglobate glands are not yet developed on the chyliferous system, they are already perceptible on the lymphatics, especially in the neck.
The chyliferous system of the mammalia, though more developed than that of all the inferior classes, is still imperfect as a hy draulic apparatus when compared with the sanguiferous system. The lacteal and lym
phatic systems may still be regarded as mere appendices of the venous, performing the func tions which are assiomed to veins in the inver tebrated classes, arid serving as inlets to the materials which renovate the blood. No pul sating sacs have yet been detected in the lym phatic system of quadrupeds, nor any distinct motion in the lacteals, the receptaculum, or the thoracic duct. The chyliferous system of this class presents a superiority of develop ment in the almost sanguineous characters of the chyle, in the more perfect structure of the vessels and their valves, in the development of the conglobate mesenteric glands, in the frequent unity or concentration of the thoracic duct, and in the more isolated condition of this system from the sanguiferous. • The mesenteric glands are chiefly confined to the mesentery of the small intestine ; they are generally placed apart. from each other ; sometimes they are , united into a pancreas Asellii ; they are firm in texture, highly vascular, and composed of con voluted lacteals, like more concentrated forms ; of the plexuses of the lower vertebrata.
? 7.? /2 1