The Mosaic History of the Creation of the World / Illustrated by Discoveries and Experiments Derived from the Present Enlightened State of Science; With Reflections, Intended to Promote Vital and Practical Religion

by Thomas Wood

These different modifications, in the respiratory organs of the higher and lower orders of animals, are all formed with the same intention, viz. that the blood may be exposed more or less to atmospherical air. In consequence of this the blood undergoes a process similar to combustion, which extracts from it a part of its carbon, in the form of carbonic acid, and by this means increases the relative proportion of its remaining elements. The inspired air at the same time is deprived of a part of its oxygen, which is the elastic fluid which commonly supports respiration. All the corresponding effects produced upon the blood are not yet fully explained. But by this means the color of the blood is changed from a dark to a florid red, it acquires the power of exciting the action of the heart, and is fitted for its various purposes within the body.[195] By these organs, respiration is performed more or less extensively in the different orders of animals, corresponding in a great degree, to their activity, digestive powers, and the heat maintained in their bodies. Birds, whose extensive respiratory organs consume a larger quantity of air, are capable of greater exertion; make more frequent meals than quadrupeds, and maintain a superior temperature. Quadrupeds hold a middle place between birds and reptiles. Respiration appears in the class of reptiles, as Frogs and Toads, to be a subordinate function only; they can exist without it nearly as long as they please; at the same time they make very long fasts, and the heat of their bodies is more variable and lower than quadrupeds; hence they are called cold blooded animals. Their other habits accord well with their organs of respiration. They generally live in impure air, their motions are languid, and they pass a great part of their existence in a state of torpidity.

A subordinate use of respiration in most animals, is the formation of the voice: for this purpose there are membranes stretched across the narrow part of the windpipe, which are thrown into a state of vibration by the current of air: the vibrations thus produced, being modified by other accessory parts, produce the voice. In many animals, however, it is produced by a very different mechanism. Some animals employ the friction of certain elastic parts of the body, as Grasshoppers and Crickets; others employ the vibration of certain parts in the air, whilst others impress a rapid motion on portions of air inclosed in certain parts of their bodies.

There is a particular part of the heart in man, intended merely to propel the blood, which passes through the lungs to receive the influence of the air; this is the right ventricle; from whence the blood passes, by the pulmonary artery, through the minute vessels expanded on the air cells, and is changed from a dark to a florid color: it is then returned back to the left ventricle, by the pulmonary veins, and is propelled over the rest of the body, where it is again changed (by the abstraction of certain properties) to the dark color peculiar to venous blood: the blood is lastly conveyed by the veins to the right side of the heart from whence it set out, having passed through two circles.

The blood thus prepared by the lungs for circulation, passes in different quantities to different parts of the body, according to their activity, and has various fluids formed from it, which are called secreted fluids, as gastric juice, milk, bile, &c. The parts of the body forming many of these fluids, are very peculiar in their structure, and are called glands. They consist in an arrangement of vessels, endowed with a mode of action, with which we are unacquainted, by which the component parts of the blood are disposed to enter into new combinations, and to form compounds differing from the blood itself. Thus the vessels are arranged on the inside of the stomach, in such a way, as by their action to form gastric juice from the blood; on the same principle, milk is produced from the blood which circulates in the breast, or bile in the liver. As gastric juice, milk, and bile, differ very much from each other in their properties, we must infer, that there is a considerable variety in the action, by which these vessels form these fluids from the blood; and this is necessarily connected with a variety in arrangement, which is the case in all the glands of the body. In one gland, for example, the blood-vessels form a minute net-work; in another, are convoluted at their extremities; in a third, a large branch suddenly divides into a number of small branches, like the hairs of a painter’s brush; in a fourth, they are disposed in an arborescent form, each gland differing from every other in the mode of distribution of its blood-vessels, and forming different products from the blood.

The substances formed by many of the glands of the body, are applied to useful purposes, within or without the body. An instance of the former we have in the bile formed by the liver, or the gastric juice formed by the stomach; and of the latter, in the milk.—Other secreted fluids are rejected as excrementitious: the best example of this is the urine formed by the kidneys. This gland separates from the blood a great variety of substances, which might otherwise prove noxious by circulating along with it; many of these have occasionally very curious chemical properties, and under a certain state of the body, the altered secretion of this organ is very remarkable, in as far as it produces a large quantity of a familiar substance, which in this instance is composed within the body. In the disease called diabetes, for example, a patient sometimes makes four or five gallons of urine in the 24 hours, in which is dissolved a considerable quantity of matter, like common sugar or treacle, probably to the amount of two or three pounds.

Besides these fluids formed from the blood, each by an appropriate glandular apparatus, there are watery fluids constantly secreted in various parts of the body; and, that these may not accumulate, or remain after they have performed their office, it is necessary for the body to be furnished with vessels, whose powers of removal may keep pace with the deposition of these fluids. This introduces the system of vessels called absorbents, which are distinct in their office and nature from the blood-vessels, and are widely diffused over the whole body. In every part of the body a limpid fluid is thrown out for the purpose of easy motion, moistening the cellular membrane, which connects the various parts of the body to each other, and lubricating the contents of all the cavities of the body; this fluid is thrown out in the form of vapor by the exhalents, which belong to the arterial system, whilst the lymphatic absorbent vessels, by their action, remove what is not convenient for the function of the part; and these two actions, of deposition, by the exhalents, and absorption, by the lymphatics, go on during health, so nicely balanced, that when we open into any of the great cavities of the body, as the belly or chest, the quantity of fluid we find is extremely small. When, however, the balance between these two orders of vessels is destroyed, when the exhalents throw out more fluid than usual, and the lymphatics only absorb their natural quantity; or the exhalents deposit their natural quantity, whilst the lymphatics absorb less than natural, accumulation of water in the cellular membrane, or great cavities of the body, takes place, and produces dropsies.

There is another set of vessels, which have been already mentioned, a part of the same system of absorbents, which from their office of absorbing a white fluid, the chyle, have been denominated lacteals; these arise from the inner surface of the intestines, in great numbers, and convey the chyle into the general mass of blood.—Whilst the minute beginnings of the lacteal vessels, from the internal surface of the intestines, is a matter of ocular demonstration, we have only presumptive proof of the origin of the lymphatics, which make the greatest part of the absorbent system. We have, however, good grounds for concluding, that they arise from every external and internal surface of the body. We find, for example, that certain remedies, as mercurial ointment, or turpentine, rubbed on the skin of any part of the body, produce effects on distant parts; the mercury by removing affections of various parts of the body, the turpentine increasing the flow of urine, and giving it a peculiar odor: these effects are explained by presuming the absorption of these substances, by the lymphatics, arising from the surface of the skin. We have further proof of this from the occasional absorption of watery fluids, under peculiar circumstances. Sailors at sea, in want of fresh water, have quenched their thirst by dipping their clothes in salt water, and applying them to the surface of the body, from which only the elementary part was absorbed by these vessels. A jockey, after reducing himself to a great degree has become in a short time too heavy to ride his match, merely by drinking a glass of wine, which had stimulated the absorbents of the skin to take up a large quantity of aqueous matter from the air. Or a person gibbeted alive, has been observed to make a considerable quantity of urine as long as he lived, without any liquid being taken by the mouth. These are all considered as evidences that the lymphatic absorbent vessels arise from every external surface of the skin, and are capable of taking up substances applied to them.

We find next that water accumulated in the large cavities of the chest or abdomen, or underneath the skin in the cellular membrane, of every part of the body, is occasionally removed from these situations, by remedies which have the power of increasing the action of the absorbent vessels. We hence conclude, that these vessels arise from every internal part, and are, in short, widely diffused over the whole body, though their beginnings are too minute to be detected by any mode of examination with which we are acquainted.

The absorbent vessels, from whatever part they arise, terminate in the blood-vessels, principally by one vessel or trunk, which is called the thoracic duct. This commences in the cavity of the abdomen, passes through the chest on the right side of the spine, and, at length, enters a large vein situated on the left side of the neck. Through this vessel, besides the fluids taken up in various parts of the body, the whole of the nourishment from digested aliment passes into the blood; it may therefore be said to be the most important vessel in the body,[196] and it is situated in one of the safest positions in the body, so that an injury done to it is a very rare occurrence.

Thus the absorbent system is formed of two sets of vessels, having the same structure, the same absorbing office, and the same termination, but differing in the fluids they convey, and the parts of the body they occupy. The one widely diffused over the whole body, and from their office of usually absorbing limpid fluids, called lymphatics; the other arising only from the intestines, and denominated lacteals, from the milky whiteness of the chyle they absorb.