The lymph which circulates through the lymphatic vessels is an alkaline fluid composed of a plasma and corpuscles. It may be considered as blood deprived of its red corpuscles and, diluted with water. Nothing very definite is known respecting the functions of this fluid. A large proportion of its constituents is derived from the blood, and the exact connection of these substances to nutrition is not properly understood. Some excrementitious matters are supposed to be taken from the tissues by the lymph and discharged into the blood, to be ultimately removed from the system. The lymph accordingly exerts an important function by removing a portion of the decayed tissues from the body.
In all animals which possess a lacteal system there is also a lymphatic system, the one being the complement of the other. The fact that lymph and chyle are both conveyed into the general current of circulation, leads to the inference that the lymph, as well as the chyle, aids in the process of nutrition. The body is continually undergoing change, and vital action implies waste of tissues, as well as their growth. Those organs which are the instruments of motion, as the muscles, cannot be employed without wear and waste of their component parts. Renovated tissues must replace those which are worn out, and it is a part of the function of the absorbents to convey nutritive material into the general circulation. Researches in microscopical anatomy have shown that the skin contains multitudes of lymphatic vessels and that it is a powerful absorbent.
Absorption is one of the earliest and most essential functions of animal and vegetables tissues. The simpler plants consist of only a few cells, all of which are employed in absorption; but in the flowering plants this function is performed by the roots. It is accomplished on the same general principles in animals, yet it presents more modifications and a greater number of organs than in vegetables. While animals receive their food into a sac, or bag called the stomach, and are provided with absorbent vessels such as nowhere exist in vegetables, plants plunge their absorbent organs into the earth, whence they derive nourishing substances. In the lower order of animals, as in sponges, this function is performed by contiguous cells, in a manner almost as elementary as in plants. In none of the invertebrate animals is there any special absorbent system. Internal absorption is classified by some authors as follows: interstitial, recrementitial, and excrementitial; by others as accidental, venous, and cutaneous. The general cutaneous and mucous surfaces exhale, as well as absorb; thus the skin, by means of its sudoriferous glands, exhales moisture, and is at the same time as before stated, a powerful absorbent. The mucous surface of the lungs is continually throwing off carbonic acid and absorbing oxygen; and through their surface poisons are sometimes taken into the blood. The continual wear and waste to which living tissues are subject, makes necessary the provision of such a system of vessels for conveying away the worn-out materials and supplying the body with new.
CHAPTER VI.
PHYSICAL AND VITAL PROPERTIES OF THE BLOOD.
