Popular Science Monthly/Volume 19/August 1881/The Blood and its Circulation I
|THE BLOOD AND ITS CIRCULATION.|
THE main facts of blood circulation have been known only two hundred and fifty years. This would be surprising if we were not aware that most of our certain knowledge in natural history, including many truths of easier discovery than the circulation of the blood, has been gained within the last one hundred years. And, indeed, the blood and its movements are not yet fully understood. Several points which, at first thought, would seem of easy solution, are matters in dispute or confessed mysteries. The purpose of this article is, not to publish new truth or discuss difficult points, but to compactly present the fundamental and interesting facts relating to the circulation in all animals.
The necessity of a circulating nutritive fluid lies in the localizing of the process of digestion. In proportion as digestion and absorption of food become specialized and restricted to certain parts, circulation becomes more important in order to convey that food to the tissues, and carry from the tissues the worn-out material. To maintain the character of the fluid, it must itself undergo constant change, and hence the excretory processes—respiration being the most urgent—which increase the necessity for movement of the fluid. Circulation of the nutritive fluid is the immediate function for upbuilding and repairing the body. It harmonizes the several vegetative functions, and should be regarded as the primary function, to which all the others are subservient.
The amœba, sponge, and tapeworm have no blood; they have no necessity for it, as they are destitute of digestive organs, their food being in immediate contact with all parts of the body: or, we might regard their blood as simply the water or fluid in which the animal is immersed. In animals possessing the simplest digestive cavities, as the jelly-fish and sea-anemone, the blood is merely the dissolved food, corresponding to the chyme of higher animals. In the starfish, sea-urchin, and other invertebrates, having a complete and distinct stomach, the blood is chyle; while in vertebrates the blood is a distinct fluid, chemically very complex, difficult of analysis, and not perfectly understood: structurally, it is essentially the same in all animals—a clear fluid containing organic particles.
The blood contains all the nourishment which supports the various tissues of the whole structure. It may properly be regarded as the fundamental tissue, and is well named in the French chair coulant—running flesh. It changes rapidly by eating, exercise, and any influence which affects the supply of nutriment or the waste of the body. It is derived primarily from the new food, received in the higher animals chiefly throuarli the lacteals and veins of the stomach: secondly, from the waste of the body received through the lymphatics and thoracic duct; and, thirdly, through respiration, which supplies oxygen. The amount of solid matter seems to bear a proportion to the amount of flesh in the diet and to the temperature of the animal, being greater in the carnivorous and warm-blooded animals.
In color, the blood of all vertebrates is red, excepting that of the
amphioxus, the lowest animal of the sub-kingdom, which is colorless. In the muscles of fishes it is also white. In the invertebrates the blood is of various colors, but commonly white, on account of which fact they were formerly supposed to be destitute of blood.
Microscopic examination of the blood of a vertebrate animal shows Fig. 2.—Blood-Corpuscles (relative size) a, Man; b, Blenny; c, Frog; d, Newt.that the color is due to an immense number of red particles floating in a watery fluid. But the shape and size of these corpuscles vary in the different groups of vertebrates, and in different species. In man, and all mammals excepting the camel tribe, the red corpuscles are biconcave disks. In the camel they are elliptical. The corpuscles in all other vertebrates are nucleated, or have a thickened center. Those of birds, reptiles, and amphibians are elliptical, while those of fishes are discal, elliptical, or angular.
The size of the red blood-corpuscles bears little relation to the size of the animal, except within the natural groups, as the orders of mammals and the class of birds. The largest are found in the amphibians, those of the proteus being 400 of an inch in diameter. The smallest are found in the musk-deer, being 12000 of an inch. Those of the ostrich are 1600 of an inch, and of the humming-bird 3000 of an inch. Yet this tiniest of vertebrates equals, in the size of its blood-corpuscles, the largest of living creatures, the bulky whale. Those of man are from 3000 to 3500 inch. The value of microscopic measurements of blood-corpuscles, as evidence in legal cases, has been
much overrated. It is quite impossible to distinguish human blood from that of the dog, and, without very extensive measurements, from that of some other mammals.
These red corpuscles are frequently larger than the capillary tubes through which they have to pass, but, on account of their elasticity, they squeeze through and afterward regain their shape.
It is estimated that a drop of human blood contains one million corpuscles—a late authority says five millions—in a cubic millimetre.
In addition to the red corpuscles of the vertebrates, all true blood contains colorless corpuscles. These are nucleated in the vertebrates, mollusks, and higher articulates. They are usually smaller than the red, and not nearly so numerous. They vary rapidly in number according to changes in the body or blood, and may bear a proportion to the red of one in one thousand to one in three hundred. Although generally globular, they have no fixed shape, but have amœboid movements. Indeed, the resemblance is so close between the amœba and these white corpuscles, that Professor Huxley, in defining the amœba, says it is structurally "a mere colorless blood-corpuscle leading an independent life." And again he says: "Leaving out the contractile vacuole, the resemblance of an amœba in its structure, manner of moving, and even of feeding, to a colorless corpuscle of the blood of one of the higher animals is particularly noteworthy"; also in a foot-note to this, "contractile vacuoles have been observed in the colorless blood-corpuscles of amphibia under certain conditions." Is it possible that the human body is an aggregation or colony of low individuals, something like a sponge? It is believed that the red corpuscles are produced from the white, being only their modified nuclei. They are more numerous in the capillaries and veins. The death and reproduction
of the blood-corpuscles are rapid and constant. Dr. Draper estimates that twenty millions die at every breath. In transfusion of the blood of a bird into a mammal, the bird-corpuscles soon disappear.
Upon exposure to the air the fibrine of the blood hardens, and, entangling the corpuscles, forms the clot leaving a yellowish liquid called serum. The composition of the blood may be graphically shown as follows:
|Liquor sanguinis||Serocity water and salts|
Coagulation serves in nature the purpose of stopping wounds. It is providentially more rapid in the lower animals, as they have no artificial means of arresting the flow of flood; and quite instantaneous in insects. What prevents the blood from coagulating during life, or in the blood-tubes, is unsettled. It can be prevented by salt.
The temperature of the blood depends upon the rapidity with which the blood is oxidized; and, within natural orders, it has a relation to the activity of the animal. For example, that of the swallow is 111°, hen 109°, gull 100°; among mammals, squirrel 105°, cat 101°, dog 99°, man 98°. The animals called cold-blooded are only relatively so, for fishes and reptiles have a temperature somewhat above that of the water or air in which they live. Even the lower creatures are slightly warmer than the surrounding medium.
The weight of the blood, which is always greater than that of water, depends, of course, upon the amount of solid matter and the abundance of the corpuscles. In man, the red corpuscles constitute one third to a little less than one half the mass of the blood. The blood of birds has the largest proportion; and it appears that the temperature bears a relation to the amount of solid matter.
The amount of blood is greater in warm-blooded animals; and the proportion of blood to the size of the body increases with the size. The blood of man is by weight about one thirteenth the weight of the body. The dog has blood equal to one fifteenth its body-weight; rabbit, one eighteenth; cat, one twenty-first. The amount of blood in the elephant and the whale has not been determined; but the heart of the whale is three feet in diameter.
The operation of transfusing the blood of a living animal into the vessels of another, or of one that is dying, was known in ancient times, and has been practiced at intervals for the last three hundred years. Extravagant hopes concerning it were formerly entertained. It was believed that diseases might be cured, impaired reason restored, old age deferred, and even the dead returned to life. In late years the eminent Brown-Séquard states that a dead dog was by this means restored to life for twelve hours; but the experiment has never been confirmed, and doubtless the animal was not dead, as supposed. It is also stated that a maniac was restored to reason by the blood of a calf.
In modern medical science, the transfusion of blood has become a well-recognized operation for cases of exhaustion from simple loss of blood. For this it is frequently practiced, and with success in the majority of cases. For general weakness and disease it has sometimes been used, but has not proved reliable.
The amount of blood used in transfusion is usually a very few ounces, sometimes only one or two drachms—rarely ten or more ounces; a small quantity is safer. The blood of a different species of animal is considered dangerous when used in large quantity. Venous blood is preferred, and may or may not be defibrinated.
Pure milk has been successfully used instead of blood; and even artificial mixtures are employed. Richardson kept a monkey alive for several weeks by a daily injection of an artificial blood.
The veins of the extremities are generally selected for the operation, they being less likely to admit air, which might be fatal by causing coagulation.
From what has already been stated as the purpose of circulation, we should not expect to find any circulation in those animals which are destitute of a separate digestive cavity; and in such we do not discover a true blood-circulation. But it would appear that no animal is entirely without the power of distributing its food to the parts of the body. In the amœba this is accomplished by the movements of the protoplasmic body, whereby the portions which have enwrapped and dissolved food-particles are blended with the less nourished parts. The "contractile vesicles" of the amœba may also have to do with the distribution of nourishment, though they are usually regarded as respiratory or excretory in function.
Next to this, in simplicity, is the prolongation of the digestive cavity for the distribution of food. This is found in various animals of different classes. The jelly-fish has a system of four canals, radiating from the imperfect stomach, and uniting with a circular canal at the margin of the body. We may regard this as the earliest development of organs for conveying nutriment. A similar condition exists in the anemone; and spiders have prolongations of the stomach in addition to their circulating organs.
|Fig. 5.||Fig. 6.|
True circulation is found only with a complete separation of the digestive cavity from the visceral or general body cavity. In many invertebrates there is simply a flux and reflux of nutritive fluid in this visceral cavity, but no special circulating vessels. This is the condition in the bryozoa, the lowest of mollusks, in the rotifera, and in the larvae of certain myriapods and insects. In these the fluid is more the nature of chyle, and is called the chylaqueous fluid.
The "circulating system is gradually developed as an offset of the visceral cavity." This is shown by the low ascidian mollusks, in which there is a sinus system prolonged from the visceral cavity, but freely communicating with it. In this system the chylaqueous fluid flows alternately in either direction, being propelled by a pump or rudimental heart, which is only a muscular portion of one of these sinuses. The same condition is found in some low crustaceans and arachnids, and in the larvæ of certain insects.
In former articles the sea-urchin has been noticed as the lowest Fig. 7.—Diagram of Sea-Urchin. a, Anus; b, Stomach; c, Mouth; e, Heart, which by vascular rings encircles the alimentary canal at d and f. possessor of true teeth and stomach, and we now have to award it the added honor of the first distinct heart and blood-vessels.
The more highly organized invertebrates have a muscular heart and true arteries. But the blood always enters the visceral cavity before returning to the heart. In other words, there is no closed current in the invertebrates, the system of circulating vessels being in direct communication with the body cavity. Regarding the circulation of the lower invertebrates, there is still much uncertainty, as various sets of vessels are found in different groups, the purpose of which is obscure, and their relation to the blood-circulation a matter of investigation. The great variety in the circulation of the many groups of invertebrates renders a detailed description impossible. It will be consonant with the present purpose to briefly describe only a few typical forms.
The typical system of the articulates is simply a segmented vessel
lying lengthwise in the back of the animal. This dorsal tube, trunk, or "heart," is open at both ends, and has openings along the sides, guarded by valves. The chylaqueous fluid fills the body cavity, bathing the heart and all the viscera. A puncture of the skin alone allows the blood to issue. The walls of the tubular heart are muscular and pulsatile. When the heart expands, the nutritive fluid is drawn in at the hinder end and lateral apertures, and upon contraction it is forced forward and escapes at the forward end, being prevented by valves from flowing backward or escaping laterally. The fluid finds its way backward through the lacunae or passages between the tissues and viscera. The dorsal vessel prevents the stagnation of the fluid.
In the myriapods the dorsal trunk has as many segments as there are joints of the body. One of the millepeds has not less than one hundred and sixty. Centipeds have generally twenty-one segments, and besides the pair of valves for each joint there is given off a pair of arteries. These unite to form a ventral tube. Insects have the heart segmented only in the abdomen, and never more than eight segments. An arterial prolongation of the trunk as a simple tube extends to the head.
As spiders and scorpions have localized breathing-sacs, the require Fig. 9.—Diagram of Mollusk. a, Alimentary Canal; h, Heart; n, n’, n", Nervous Ganglia. a respiratory circulation. This is secured, not by special tubes, but by the passage of the blood, on its return to the heart, through venous sinuses or special passages between the internal organs.
The best heart among articulates is possessed by the crustaceans, the largest, though not the highest, animals of the sub-kingdom. Crabs and lobsters have a concentrated heart, a short, fleshy sac, with great propelling power, which sends the blood by several branching arteries to the parts of the body. We now find a concentration of the power which had been previously diffused in a long tube.
As the mollusks are mostly so sluggish that their circulation has little aid from the movements of the body, they require a more powerful pump. In the higher mollusks the heart has generally two cavities—an auricle for receiving the blood and a ventricle for propelling it. The bivalve mollusks have generally two auricles. In the mollusks we discover a well-developed capillary system, but the venous or return circulation is still partly lacunar. The heart of invertebrates is always systemic—it forces the blood to the body, not to the breathing-organs. But some of the cephalopod mollusks, the so-called devil-fishes, have contractile cavities at the bases of the gills, which act the part of a pulmonary heart, forcing the blood through the breathing-organs on its way to the true heart. These accessory hearts are called branchial hearts.
[To be continued.]