This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
1. Absorption is another function of the animal body, immediately connected with nutrition. By it is meant that process by which food and drinks, designed for the nourishment and growth of the body, are taken up and carried into the blood ; and also those particles and materials that have been already deposited, and have become either useless or injurious, are conveyed into the general mass of the circulating fluids, and thus removed from the system. The first is effected by the lacteal vessels; the second by the lymphatics.
2. The absorbent system, so called, consists of the lymphatic vessels, the lymphatic glands, and the thoracic duct. The lymphatic vessels arise, not only from all the mucous surfaces, but also from the whole surface of the body; the intimate tissue of every structure ; and from all cavities, such as the chest, abdomen, the joints, the pericardium, and even the ventricles of the brain. They are exceedingly small at their origin, but by uniting, form larger and larger trunks as they proceed, which is generally in the course of the veins, till they finally discharge their contents, either into the thoracic duct, or some of the large veins near the heart. Throughout their whole extent, they are provided with numerous valves, which, when they are distended with lymph, causes them to resemble a string of beads.
3. Every part of the body is supposed to be furnished with absorbent vessels, with the exception of the nails, the hair, the cuticle, and the enamel of the teeth. And even in these, it is not impossible that they may exist; only they are too small to be detected.
4. The lymphatics of the small intestines, called lacteals, are the agents of digestive absorption. They arise from the surface of the mucous coat, pass between the serous and muscular coats, and proceed to the small glands or ganglions of the mesentery. As they emerge from these, they increase rapidly in size, till they finally unite in a large trunk, which passes up along the spine, and at last empties its contents into the left subclavian vein near the heart. Many physi. ologists are of opinion that the lacteals not only terminate in the thoracic duct, but also in numerous veins in the abdomen.
5. The chyle, which is the fluid taken up by the lacteals, does not exist ready formed in the chyme, but is formed or manufactured out of the nutrient principles contained in it, by a specific action of the lacteal vessels themselves. In like manner, the sap which is contained in vegetables, does not previously exist ready made in the materials which are absorbed from the ground, but is formed by the peculiar action of the roots. No chyle has ever been found in the intestines.
6. As the chyle passes on towards the heart, it undergoes important changes. It loses some of its albuminous qualities ; while its fatty matter, its fibrin, and cruor, considerably increase. Its tendency to coagulate, also becomes greater as it approaches the venous system, and it becomes clearer and more transparent. What is the precise nature of the change which the chyle undergoes in passing through the glands is unknown.
7. Absorption not only takes place from the small intestines, but from the whole tract of the intestinal canal, including the mouth and aesophagus. But those vessels which absorb chyle, are chiefly found in the small intestines. It is highly probable that alimentary substances may be directly absorbed from the intestines, without undergoing any pre. vious change or assimilation, like alcohol or water ; but that in their passage through the absorbent system, they undergo a species of digestion, and become in a good degree fitted for the replenishment of the blood. To this, however, alcohol is an exception, as it frequently, if not always, passes into the blood unchanged.
8. Various medicinal substances are absorbed, and enter the circulation, nearly or wholly unchanged. Colouring matter, such as madder, is taken up and carried into the blood, and even tinges the bones. Oderiferous particles, such as of garlic, camphor, asparagus, etc, are also readily absorbed, and in this way the blood derives its saline properties. It is pretty well established, that articles not of an alimentary nature, which are absorbed, are chiefly taken up by the veins, while the lacteals absorb chyle more readily than any other substances.
9. Absorption takes place from the external surface or skin. This is proved by many facts. Thirst may be quenched by applying moist cloths to the skin, or by bathing. The body increases in weight by the use of the bath; and it has been found that the hand, immersed to the wrist in warm water, will absorb from 90 to 100 grains of fluid in the space of one hour. The saliva has become bitter by the absorption of sea water ; and it is stated that patients have been supported by baths of milk or broth.
10. Medicinal substances are often absorbed by the skin. Mercury, Spanish fly, morphine, and many other articles are frequently introduced into the system through the skin. Metallic quicksilver has been found in the bones of persons who had been subjected to mecurial frictions; and it has also been obtained by distilling the blood of rabbits, dogs, and cats, which had been rubbed with this mineral. Gases are also absorbed by the skin.
11. As every part of the body is subject to constant renovation and change, absorption must be continually going on among the particles and substance of which each organ is composed. This is called interstitial absorption. It is this which counterbalances the action of the nutrient vessels, and preserves the form and size of every part of the body. When it is too active, the body emaciates ; when it is deficient, plethora is the result. In the later periods of life it is more active than nutrition, and the body dwindles in size ; in youth the reverse is the case.
12. Foreign bodies, introduced into the substance of organs, are also absorbed. Wens and tumours of considerable size, often disappear from the same cause. Instances are known where the absorbent vessels have set to work and removed the whole bone of a limb j and but lately a case occurred in Boston, in which every particle of bone in the arm was thus taken up and carried away, after a fracture had occurred. How admirable is that arrangement by which the vessels of absorption and secretion so act as to balance each other; and how soon would a loss of this balance produce derangement, disease, and even death.
13. Another form of absorption is called respiratory, which we have already considered under the subject of respiration. We understand by it, merely the introduction of oxygen into the blood, through the pulmonary cells. Substances, however, in a state of vapour, or fine dust, are also readily imbibed when drawn into the lungs; such as metallic vapours, oderiferous particles, marsh and other effluvia. It is in this way, probably, that contagious diseases are caught.
14. It is by means of what physiologists call recrementi tial absorption, that fluids are removed from the system which are secreted upon surfaces that have no external outlet ; as in the cavity of the chest, abdomen, brain, etc. These fluids are various : such as the serous fluids ; the sy novia of the joints ; the fat; the marrow, and the humours of the eye. It is this which prevents dropsies in these various cavities ; and also removes them when already existing. This form of absorption is also proved by the fact, that foreign substances, placed in contact with these surfaces, in a short time disappear.
15. Another variety of absorption has been called excre mentitial; as it relates to the fluids which have been excreted.
These are liable to be absorbed; at least the more fluid parts of them, by which they are again carried into the mass of the circulating fluids, and such parts selected as are fit for the uses of the animal economy, such is the case with the fluids exhaled by the skin ; the mucous membranes; the saliva ; the bile; the gastric and pancreatic fluids ; the milk, etc. Thus, has it been remarked, does nature choose to subject the materials of decomposition to a careful revision, before rejecting them finally from the body.
16. All these varieties of absorption are constantly going on from the moment of birth to that of dissolution, and all the fluids which are absorbed, are changed in their character, and fitted to repair the wastes of the system. Thus, by absorption in the lungs, oxygen is converted into one of the elements of the blood; in the intestines, chyme is changed into chyle; in the tissues of the organs, solid particles are converted into fluid lymph; and from the cavities secreted and excreted fluids are again taken up and thrown into the circulation, to be once more revised and elaborated.
17. Frogs, and several other amphibious animals, are fur nished with large receptacles for the lymph, situated immediately under the skin, which exhibit distinct and regular pulsations like those of the heart. The use of these lymphatic hearts is evidently to propel the lymph along the lymphatic vessels. The frog has four of these organs; the two posterior being situated behind the joint of the hip ; and the two anterior ones on each side of the third vertebra. These organs have also been discovered in the toad, salamander, and lizard.
What is meant by absorption ? What composes the absorbent system ? Where do the lymphatic vessels arise ? Describe their structure. Are they found in every part of the body ? What are the agents of digestive absorption ? Describe the lacteals. What is chyle ? Does it exist ready formed in chyme ? What changes does it undergo in its passage to the heart ? Where are the chyliferous vessels chiefly found ? Are alimentary substances absorbed without be ing previously changed into chyle ? Is alcohol digested ? Are medicinal substances absorbed ? How is this proved ? Does absorption take place from the skin ? What is interstitial absorption ? What is emaciation owing to ?-plethora ? Are foreign bodies absorbed when introduced into the substance of organs ? Are the bones ever absorbed ? What is respiratory absorption ? What is recrementitial absorption ? What excrementitial 1 Are these processes always going on ? What peculiarity is there in the lymphatic system of frogs ?