388 GENERAL ANATOMY OF LYMPHATICS. 



of the superficial fascia, as in the hollow of the ham and groin in the 

 lower extremity, and upon the inner side of the arm in the upper ex- 

 tremity. 



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 upon 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 subclavian 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 ani- 

 mal 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 um- 

 bilical 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 border 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 contrac- 

 tile power in the valve itself ; and sometimes instead of being circular 

 the aperture is elliptical, and the arrangement of the flaps like that of 

 the ileo-coecal valve.* These peculiarities are most frequently met 

 with at and near the anastomoses of the lymphatic vessels. The 

 valves are most numerously met with near the lymphatic glands ; next 

 in frequency they are found in the neck and upper extremities where 



* Mr. Lane, loc. cit. 



