38 HUMAN PHYSIOLOGY. 



containing leucocytes. The lacteal vessel possesses a thin, but distinct 

 wall, composed of endothelial plates, with here and there openings, which 

 bring the interior of the villus into communication with the spaces of the 

 adenoid tissue. 



The structure of the larger vessels resembles that of the veins, consisting 

 of three coats 



I. External, composed of fibrous tissue and muscular fibres, arranged 

 longitudinally. 2. Middle, consisting of white fibrous and yellow elastic 

 tissue, non- striated muscular fibres, arranged transversely. 3. Internal, 

 composed of an elastic membrane, lined by endothelial cells. 



Throughout their course are found numerous semilunar valves, looking 

 toward the larger vessels, formed by a folding of the inner coat and 

 strengthened by connective tissue. 



Lymphatic Glands consist of an external fibrous covering, from the 

 inner surface of which partitions of fibrous tissue, the trabeculce, pass into 

 the substance of the gland, forming a stroma or network, in the meshes of 

 which are found the true lymph corpuscles. 



The lymphatics which enter the gland are called the afferent vessels ; 

 those which leave it, the efferent vessels. 



The Thoracic Duct is the general trunk of the lymphatic system, into 

 which the vessels of the lower extremities, of the abdominal organs, of the 

 left side of the head and left arm empty their contents, it is about twenty 

 inches in length, arises in the abdomen, opposite the third lumbar vertebra, 

 by a dilatation, the receptaculum chyli; ascends along the vertebral column 

 to the seventh cervical vertebra, and terminates in the venous system at 

 the junction of the internal jugular and subclavian veins on the left side. 

 The lymphatics of the right side of the head, of the right arm and the right 

 side of the thorax, terminate in the right thoracic duct, about one inch in 

 length, which joins the venous system at the junction of the internal jugular 

 and subclavian on the right side. 



Lymph is a clear, transparent fluid, slightly alkaline, having a saline 

 taste and a specific gravity of 1.022. It is found in the lymphatic vessels 

 throughout the body. 



Lymph contains a number of corpuscles (the leucocytes) resembling the 

 white corpuscles of the blood, which increase in number as it passes through 

 the lymphatic glands. They are about ^^ of an inch in diameter and 

 somewhat granular; they are discharged into the blood, but their function 

 is obscure. When withdrawn from the vessels, lymph undergoes sponta- 

 neous coagulation, separating into serum and clot, as in the case of the blood. 



