352 GENERAL ANATOMY OF LYMPHATICS. 



faces.* The vasa efferentia escape from the gland on the opposite, but 

 not unfrequently on the same surface ; they consist, like the vasa inferentia 

 at their junction with the gland, of several small vessels which unite after 

 a course of a few lines to form from one to three trunks, often twice as 

 large as the vasa inferentia. 



Lymphatic vessels admit of a threefold division, into superficial, deep, 

 and lacteals. The superficial lymphatic vessels, on the surface of the 

 body, follow the course of the veins, and pierce the deep fascia in conve- 

 nient situations, to join the deep lymphatics On the surface of organs 

 they converge to the nearest lymphatic trunks. The superficial lymphatic 

 glands are placed in the most protected situations of the superficial fascia, 

 as in the hollow of the ham and groin in the lower extremity, and on the 

 inner side of the arm in the upper extremity. 



The deep lymphatics^ fewer in number and somewhat larger than the 

 superficial vessels, accompany the deeper veins ; those from the lower 

 parts of the body converging to the numerous glands seated around the 

 iliac veins and inferior vena cava, and terminating in a large trunk situated 

 on the vertebral column, the thoracic duct. From the upper part of the 

 trunk of the body on the left side,, and from the left side of the head and 

 neck, they also proceed to the thoracic duct. Those on the right side of 

 the head and neck, right upper extremity, and right side of the thorax, 

 form a distinct duct which terminates at the point of junction of the sub- 

 clavian with the internal jugular vein on the right side of the root of the 

 neck. 



The lacteals are the lymphatic vessels of the small intestines ; they have 

 received their distinctive appellation from conveying the milk-like product 

 of digestion, the chyle, to the great centre of the lymphatic system, the 

 thoracic duct. They are situated in the mesentery, and pass through the 

 numerous mesenteric glands in their course. 



Lymphatic vessels are very generally distributed through the animal 

 tissues ; there are, nevertheless, certain structures in which they have 

 never been detected ; for example, the brain and spinal cord, the eye, 

 bones, cartilages, tendons, the membranes of the ovum, the umbilical 

 cord, and the placenta. The anastomoses between these vessels are less 

 frequent than between arteries and veins ; they are effected by means of 

 vessels of equal size with the vessels which they connect, and no increase 

 of calibre results from their junction. The lymphatic vessels 'are smallest 

 in the neck, larger in the upper extremities, and larger still in the lower 

 limbs. 



For the purpose of effecting the movement of their fluids in a proper 

 direction, lymphatic vessels are furnished with valves, and it is to these 

 that the appearance of constrictions around the cylinders of the vessels, at 

 short distances, is due. Like the valves of veins, the valves of lymphatic 

 vessels are each composed of two semilunar flaps attached by their convex 

 oorder to the sides of the vessel and free by their concave border. This 

 is the general character of the valves, but, as in veins, there are exceptions 

 in their form and disposition ; sometimes one flap is so small as to be 

 merely rudimentary, while the other is large in proportion ; sometimes 

 the flap runs all the way round the tube, leaving a central aperture which 

 can only be closed by a contractile power in the valve itself; and some- 



* See Mr. Lane's article on the " Lymphatic System," in the Cyclopcedia of Anatomy 

 zia Physiology. 



