Lymph (Lat. lympha, clear, pure water, from Gr. a water spirit), the nearly transparent and colorless fluid found in the lymphatic or absorbent vessels extensively distributed over the body, in nearly all the organs and tissues. The lymphatic vessels commence in the substance of the tissues, probably by minute plexuses, converge toward the central parts uniting with each other into larger branches, which usually follow the same course as the corresponding blood vessels, pass through a series of small solid glandular organs, the "lymphatic glands," and finally empty into the venous system by two main trunks, viz.: the "thoracic duct," bringing the lymph from the lower extremities, the trunk, left upper extremity, and left side of the head and neck, which opens into the left subclavian vein; and the "right lymphatic duct," bringing the lymph from the right upper extremity and the right side of the head and neck, which opens into the right subclavian vein. According to Robin, the lymphatic vessels at their commencement are closely in contact with the capillary blood vessels, so much so that the lymphatic often embraces the capillary blood vessel for one half, two thirds, or even three fourths of its circumference.
It is evident that the lymph moves in the lymphatic vessels always in one direction, namely, from the circumference toward the centre, and does not like the blood return again in the opposite direction. It is a fluid taken up by absorption at the periphery, thence carried inward toward the centre of the circulation, and finally mingled with the venous blood at a short distance from the heart. - The fluid contained in the lymphatic vessels of the intestine during digestion has received a distinct name, that of "chyle," since it differs from the lymph in general by its opaque white color, and by containing an abundance of molecular fat and a larger proportion of albuminous matters. The composition of both lymph and chyle, and the difference between them, are shown in the following analysis, by Dr. G. O. Rees, of the lymph and chyle from the ass:
The lymph and the chyle are not therefore to be considered as two distinct fluids; since chyle is only the lymph of the intestine, which during the digestive process has absorbed an unusual proportion of nutritive materials. The lymph also contains, in addition to the above ingredients, small quantities of urea, sugar, and albuminose, the two latter varying in amount with the part of the body from which the fluid is taken and the period of the digestive process. The lymph, like blood, coagulates soon after it is removed from the vessels, owing to the fibrine which it contains, forming a more or less colorless and transparent solid clot and fluid serum. It contains a very small number of round, white, granular corpuscles, similar to the white globules of the blood, but of smaller average size. When taken from the thoracic duct of the living animal it also always contains a certain proportion of red blood globules, sufficient to give to its clot a slight rosy tinge after it has been exposed for a short time to the air; but this is believed by some authorities to be owing to an accidental rupture of some of the small blood vessels connected with the lymphatic system. - The quantity of lymph discharged daily into the venous system is very considerable.
In the dog, the fluid discharged from the thoracic duct and collected by means of a silver canula inserted into its extremity, at various periods after feeding, is on the average 1'75 part per hour for every 1,000 parts of the entire weight of the animal, making 42 parts in 1,000 for the whole 24 hours. In a dog weighing 30 lbs. this would give 1 1/4 lb. of lymph and chyle daily. In a young kid weighing 14 lbs., 540 grains of lymph may be drawn from the thoracic duct per hour, representing rather more than I 3/4 lb. in 24 hours. M. Colin, of the veterinary school of Alfort in France, obtained from the thoracic duct of an ox in 24 hours more than 80 lbs. of fluid, and from a young bull a little more than 100 lbs. in the same time. In the horse, according to the same experimenter, the quantity is less than in the ruminating animals; but even in the horse he estimates the daily quantity at from 40 to 50 lbs. per day, or about 4 1/2 Per cent. of the entire weight of the animal. This corresponds with the results above mentioned as obtained in the case of the dog; and applying these lower estimates to the human subject, for a man weighing 140 lbs., it would give from 6 to 6 1/2 lbs. of lymph and chyle per day.
This quantity indicates the activity of the absorption by which the lymph is taken up from the tissues and returned by a circuitous route to the venous circulation.