308 FUNCTIONS OF NUTRITION. 



cells, which may be brought into view, like those of the capillary blood- 

 vessels, by the staining action of silver nitrate. 



Origin and Course of the Lymphatic Vessels. So far as the origin 

 of the lymphatic vessels has been demonstrated by injections, they 

 commence by irregular plexuses. They are more abundant in organs 

 which are well supplied with blood-vessels, and are absent in non- 

 vascular tissues, such as those of the cornea, the vitreous body, and 

 the epidermic and epithelial layers of the skin and mucous membranes. 

 According to Recklinghausen, the meshes of the lymphatic plexus are 

 usually intercalated between those of the capillary blood-vessels ; so 

 that the point of junction of two or more lymphatics is in the mid- 

 dle of the space surrounded by the adjacent blood-vessels. Thus the 

 lymphatic capillary is situated at the greatest possible distance from 

 the nearest capillary blood-vessels ; and in the transudation of fluids 

 from one to the other, the intervening tissue is completely traversed 

 by the nutritious ingredients of the blood. In membranous expan- 

 sions presenting a free surface, as in the skin and jnucous membranes, 

 the capillary blood-vessels are situated near the surface, while the 

 lymphatics occupy a deeper plane. In the villi of the intestine, the 

 network of blood-vessels is immediately beneath the epithelial layer, 

 and the lacteal vessel in the central part of the villus. 



Beside the lymphatic capillaries proper, certain irregularly-shaped 

 spaces or canals, containing a colorless serous fluid, have been found 

 Jin organs composed of dense connective tissue, like the central tendon 

 of the diaphragm and muscular fasciae. They are generally demon- 

 strated by treating the tissues with silver nitrate, which stains the solid 

 portions of a 'dark color, but leaves the capillary vessels and serous 

 canals uncolored. These interstitial canaliculi are regarded by some 

 observers as continuous with the lymphatic capillaries, and as the 

 immediate sources of supply for the lymph. They are distinguished 

 from the lymphatic capillaries by their smaller size, and by the fact 

 that they are not provided with an epithelial lining. 



From their plexuses of origin the lymphatic vessels pass inward 

 to the great cavities of the body, uniting into branches and trunks, 

 and following generally the course of the principal blood-vessels. 

 Those of the lower extremities enter the abdomen and join the abdomi- 

 nal lymphatics, to form the commencement of the thoracic duct. This 

 duct ascends, through the chest, to the root of the neck, where it is 

 joined by lymphatics from the left side of the head and the left upper 

 extremity, and terminates in the left subclavian vein, at its junction 

 with the left internal jugular. The lymphatic vessels coming from the 

 right side of the head and neck and the right upper extremity form the 

 right lymphatic duct, which terminates in the right subclavian vein 

 at its junction with the right internal jugular. Thus the lymph, 

 collected from the vascular tissues of the entire body, is mingled with 

 the venous blood a*little before its arrival at the right side of the heart. 



The Great Serous Cavities are Lymphatic Lacunae. In the am- 



