Popular Science Monthly/Volume 25/October 1884/The Significance of Human Anomalies
|THE SIGNIFICANCE OF HUMAN ANOMALIES.|
EVER since the study of human anatomy has attracted any attention, variations in the arrangement of the different structures of the body have been noticed. For many centuries, the signification of these variations was not understood; and even as lately as 1840, Dr. Knox, of Edinburgh, who had the courage to state his conviction that they connected man with the lower animals, was looked upon, even by members of his own profession, as one prompted by the evil-one. In early times, when great prejudice existed against the dissection of human bodies, and animals, such as monkeys, dogs, cats, etc., were frequently used as substitutes, the similarity of some of their muscles to those which occasionally occurred in man as anomalies, forced the anatomists to remark on them as being curious coincidences, though in their published works they drew no conclusions from their occurrence bearing on the origin of man.
In the view of our present knowledge of the animal kingdom and its development, and with the acceptance of the great principle of evolution, the explanation of these variations is simple enough, viz., that they point to the fact that man has descended from some lower form, and "is the co-descendant with other mammals of a common progenitor" (Darwin).
Again, many structures which in man are merely rudiments and quite useless, nay, sometimes a source of danger, are seen fully perfected in some of the lower animals, and in them fulfill a definite purpose. The existence of such rudimentary organs (or, as Haeckel calls them, "worthless primeval heirlooms") as the ear-muscles, the appendix vermiformis in the intestines, the thyroid gland, the remnant of the third eyelid, the rudimentary tail-bones, and many others, is not satisfactorily accounted for on the theory of the plan of general unity; but if we look upon them as parts which have become functionless and atrophied from want of use, and by heredity have been transmitted from generation to generation, a bright light is thrown on the reason of their existence. In the present paper I do not intend to dwell on the significance of rudimentary organs which exist normally in man, but shall confine myself to those structures which occur as variations.
I might here mention that some parts, as for instance certain muscles of the thumb, occur in man, but not in the lower animals; these we may take as indications of the advance of man to a still higher development.
To the study of embryology we owe much in elucidating many morphological problems, and removing others from the domain of theory. By our knowledge of this most intricate subject the significance of many variations and rudimentary organs is made plain.
It has been well said that "the development of the individual is the compressed development of the race in the process of compression; some features are suppressed or modified, and others are thrown into relief." In the development of the embryo we see the history of the race, but the higher the form the more quickly does the embryo pass through those stages and transformations which are the equivalent of what is persistent in types below. In lower forms these stages are much less rapid, and in fact are true metamorphoses. The changes occurring in the development of the common frog will furnish a familiar example of this latter statement. The more we know of embryology, the more the truth of the saying that "development means descent" is apparent.
It may not be generally known that no two individuals have exactly the same anatomical structure, and that nearly every one has in him some bony prominence, supernumerary muscle, or abnormal blood-vessel, which tells the tale of his descent. During the past nine years I have been teaching anatomy, and nearly three hundred subjects have been dissected under my immediate supervision; in these I have carefully noted the variations occurring, with the result of finding that scarcely one body is perfectly normal in every part—nay, many are very abnormal, having as many as thirty to forty variations in their bones, muscles, or arteries. I have found variations to occur more frequently in negro and Indian subjects than in those of European descent. When a variation in a bone, muscle, or blood-vessel is found, the first question asked is. What is its morphology? and it is the exception not to be able to make it out; if one fails, it is concluded that our knowledge is deficient, and that the variation has a history, if we could only discover it.
Many variations are explained when an appeal is made to comparative anatomy, a science which is as yet very incomplete, but which is rapidly enlarging its boundaries. Some animals we know by their fossil remains, and in these merely their bony structure can be studied; all the soft parts are, of course, lost forever, and can only be approximately restored by our knowledge of allied existing types of the same animals. With these few preliminary remarks I shall proceed to describe, as simply as possible, some anomalies I have myself met with, and the significance of which I shall endeavor to make clear.
Osseous System.—In a skull in my possession, whose lowness of type is manifested by the narrow forehead, prominent supraorbital ridges, wide arches of bone to inclose the large masticatory muscles, the acute facial angle, prognathous jaws, and well-marked bony prominences, are two remarkable variations:
1. An Epihyal Bone.—In all human beings there is near the ear-opening a bony spine, generally about half an inch long, and which is called, from its resemblance to an ancient pen, the styloid process; the lower end of this is connected with the hyoid or tongue bone of the neck by a fibrous cord. Now, in this skull, the styloid process is not connected with the little tongue-bone by a fibrous cord, but the
styloid process is itself prolonged down to the tongue-bone and articulated with it in the fresh state. It is quite a large bone, three and a half inches long (see Fig. 1, A). This arrangement is seen in many of the lower animals, and in them the bone, which is a very important one, is called the epihyal bone.
2. At the base of the skull on the left side, behind the mastoid process, the prominent nipple-shaped process behind the ear, is a stout, bony spur, more than three quarters of an inch long, which has a downward direction, and articulates with the first bone of the vertebral column (see Fig. 1, B). This process is rarely seen in the human being, and is the only one I have met with, but it is quite the normal condition in most graminivorous and carnivorous animals, being especially well marked in the horse, pig, sheep, and goat. In them it is an important part, and gives attachment to strong muscles which move the head on the trunk. It is called the para-mastoid process, from its proximity to the mastoid.
Supernumerary Ribs.—I suppose every one is aware that the vertebral column, or backbone, is composed of many separate bones, some of which carry ribs. The backbone is made up of thirty-three bones, seven in the neck, twelve in the trunk, five in the loins; below this we have a bone called the sacrum, which consists of five vertebræ fused together; and lower down still four small bones which represent the tail-bones, called, when taken together, the coccyx, from their
supposed resemblance to a cuckoo's beak. Now, each trunk, or dorsal vertebra, has two ribs connected with it, one on each side; so there are altogether twenty-four ribs, twelve on each side; but sometimes there are more, and, when this occurs, the extra ribs are carried by the neck (cervical) or loin (lumbar) vertebræ. I have specimens in my collection of both varieties, cervical and lumbar (see Fig. 2, C), These supernumerary ribs do not occur very frequently; still, every anatomist has observed them. Their occurrence becomes more intelligible when we know that in crocodiles, birds, and the three-toed sloth, neck or cervical ribs exist normally; that in crocodiles, alligators, and some other animals, loin or lumbar ribs are never absent; and that in man traces of them exist in the muscles of the abdomen. In the human embryo, in an early stage, a rib is always seen connected with the seventh neck-vertebra, but before the fifth year of life it becomes blended with the ordinary transverse process (Fig. 2, T); occasionally, however, this rudiment goes on developing, till it becomes a more or less perfect cervical rib (see Fig. 2, C).
Supra-Condyloid Process.—It is not uncommon to find, in the humerus or arm-bone of man, a hook-like process on the inner side of the lower end, having a downward direction; this, with a band of ligament which connects its tip with the humerus lower down, forms a foramen or opening through which pass the great artery and nerve of the arm (see Fig. 3, A, B). This foramen is found in about three per
|Fig. 3.—A, the supracondyloid process of the human humerus; B, the ligament which completes the foramen. (After Struthers.)||Fig. 4.— Bones of Fore-limb of Cat. S, the supracondyloid foramen, with vessel and nerve passing through. (After Struthers.)|
cent of recent skeletons, but much more commonly in the skeletons of ancient races. In very many bodies a trace of this foramen is seen, represented by a very small bony prominence, or only by a band of fibrous tissue. In many of the lower animals it is the normal condition. It is seen in nearly all the carnivora, except the plantigrades (though it has been found in the cave-bear); it is also seen in monkeys, lemurs, and sloths. In these it is generally completed by bone, though in some by bone and ligament as in man. In the animals above mentioned it serves the purpose of protecting the great nerve and vessel of the fore-limb from pressure during flexion, and it also affords a more direct course by which these structures can supply the parts below (see Fig. 4). In man when this arrangement occurs, owing to the altered position of the limb, the nerve and blood-vessel are actually dragged out of their course to pass through this opening; so in him it serves no useful purpose. This variation is, as was first pointed out by Professor Struthers, well known to affect certain families. The only reasonable explanation of the occurrence of this structure appears to be that of reversion to the type of some mammalian ancestor in which this part was functional, or in other words served a definite purpose (Struthers).
Third Trochanter.—The third trochanter of the thigh-bone occurs about as frequently as the supra-condyloid process. On the Tipper part of the thigh-bone there are two prominences called the greater and less trochanter; a third prominence (trochanter tertius) sometimes occurs; it is situated a little below the great prominence, and gives attachment to the large muscle of the buttock (glutceus maximus). According to Fiirst, in forty skeletons of Swedes examined by him in the Caroline Institute in Stockholm, fifteen possessed this process, and, in six skeletons of Laplanders, four had a third trochanter. I have seen it in only about one per cent of the skeletons I have examined. In many of the lower animals this process is enormously developed; it is very prominent in the horse and rhinoceros, and in many others it exists in a slighter degree.
One more example from the osseous system, and I shall pass to the softer structures. In the human wrist are eight small bones called carpals, and arranged in two rows; occasionally between the two rows we have a ninth bone called the os centrale. This os centrale is always present in the higher apes and some of the rodents. We also find that in every human foetus at an early period a rudiment of this bone exists, but it has entirely disappeared by the fourth month of fetal life.
Circulatory System.—Every naturalist now admits that the various stages of development of an animal, as well as its specialized parts, are often found to correspond with permanent conditions of animals lower in the scale. A good illustration of this is seen in the development of the human heart and blood-vessels. In the early stages of development we have a heart with a single cavity, connected with a vessel at each end as in ascidians; later on the blood-vessels consist of a series of arches which go to the gills or branchial clefts as in fishes and amphibia, while the heart consists of two chambers separated by valves, and is placed far forward in the neck. The gill-arches now partly disappear, and, though the circulation still remains single as in reptiles, the heart-cavities are beginning to be separated into two distinct systems. Soon a double circulation is acquired by a complete separation of the heart into right and left. The right heart propels the venous and the left the arterial blood. At this period the condition is identical with that of birds; at last the true mammalian type of heart and blood-vessels develops and remains permanent. The arrangement of the great blood-vessels going to and from the heart varies considerably in different mammals. In man the rule is for the great artery, carrying the blood from the heart to the general system, to give off three main branches, named the innominate, left carotid, and left subclavian (see Fig. 5). These are distributed to the head and the two arms; the main vessel or aorta curves downward and distributes blood to the trunk and lower extremities. These branches are now known to be derived from certain of the original gill-arches
|Fig. 5.—Normal Aortic Arch in Man. R. C, L. C., right and left carotid arteries going to the head; R. S., L. S., right and left subclavian arteries going to the arms; I, innominate artery.||Fig. 6.—Gill Arches—the dark lines show the ones which normally persist in man.|
which persist (see Fig. 6), and when any variation in their arrangement takes place it always occurs in the line of some of these gill-arches; that is, some of the arches persist which usually are obliterated. Nearly all the variations occurring in these large vessels in man are found to be the regular condition in animals lower in the scale; for instance, sometimes only two branches are given off instead of three; each of these, again, dividing into two, one for the head and one for the arm of that side (see Fig. 7, B). This is the usual arrangement in the bat, porpoise, and dolphin. The commonest variation of the aortic arch is where the innominate gives off the left carotid, and so supplies both sides of the head (see Fig. 7, A), the artery supplying the left arm coming off as usual. This is the normal condition in apes, bears, dogs, and all the feline tribe. In some rare cases in man one branch only comes off from the aortic arch, and this, again, divides into the various arteries supplying the head and arms. In horses and other solipeds, we see this form of aortic arch (see Fig. 7, D). Again, the branches may all be given off separately from the arch, as is the arrangement in the walrus (see Fig. 7, C).
I have three times met with rather a rare anomaly of the great veins going to the heart from the upper part of the body. The usual
arrangement in man, on each side, is for the great vein of one arm and the corresponding side of the head to unite and form a single trunk (brachio-cephalic), so we have two large venous trunks, one on each side; these two trunks then join to form a single large vessel, called the superior vena cava, which empties its blood into the right side of the heart (see Fig. 8, A). It occasionally happens that the great venous trunks formed by the veins of the arm and head of each side do not unite to form the superior vena cava, but each continues its downward course and opens separately into the heart (see Fig. 8, B). On studying the development of the blood-vessels, we find that in early fetal life this condition of affairs exists, but after a time a transverse branch forms between the two trunks. This branch gradually enlarges, while the left trunk shrivels up, and at birth is only represented by a fibrous cord. This anomaly of the veins we find, then, is a persistence of a usually transient fetal condition in man, and also that in all birds and many of the lower mammals it is the permanent condition.
Muscular System.— The muscular system of man is liable to many variations, nearly all of which are interesting from a morphological point of view.
It is not uncommon to find in man useless rudiments of muscles which exist in a well-developed state in some of our more humble fellow-creatures, and in them serve a definite purpose.
In man the "skin-muscles" are very feebly developed compared with those seen in many of the lower animals. The only remnants of these in man are, the muscle which wrinkles the forehead (occipito-frontalis), the muscle immediately under the skin covering the side of the neck (platysma myoides), and the palmaris brevis, a little bundle of muscular fibers in the palm of the hand; not unfrequently remnants appear abnormally in other situations, as, over the breast (see Fig. 9), in the arm-pit, on the back, etc. The skin-muscles are well developed in those of the mammalia which have loose skins, as, for example, the hedgehog, porcupine, and porpoise. In the hedgehog, when the skin-muscles contract, the animal becomes rolled up as in a bag of muscles. The sportive gambols of a school of porpoises are effected by an abundant supply of these skin-muscles; in the horse the skin-muscle is called the panniculus carnosus, and every one who has seen a horse twitching its skin to get rid of troublesome flies will easily understand how serviceable it is to that animal.
In all human beings there is a small muscle going from a hooked process (coracoid) on the upper end of the shoulder-blade to the inner side of the arm-bone about the junction of its upper and middle third. Sometimes this muscle is continued down to the lower end of the arm-bone; or, again, it may be quite short, and attached to the bag of fibrous tissue covering the shoulder-joint. On referring to the anatomy of the lower animals, it is found that both these varieties exist normally, but in a much more highly developed state; they are especially well seen in animals which use their fore-limbs for digging, climbing, or swimming. In them the muscle is of large size, and reaches to the inner edge of the lower extremity of the arm-bone; in man, when it reaches thus far, it is only rudimentary and of no use.
Another muscle which I have seen in about three per cent of human subjects is a small one which goes from the breastbone to the upper end of the shoulder-blade. This muscle is well developed in animals which have no collar-bones; it reaches its highest development in the horse, pig, hippopotamus, and elephant. It is also seen in the Guinea-pig, Norway rat, and wombat. It is quite rudimentary when it exists in man, and serves no useful purpose.
In man, near the elbow-joint, and lying close together, are two muscles going from the upper to the lower arm; one in front (brachialis anticus), which helps to bend the elbow, and the other to the outer side (supinator longus), which supinates or twists the fore-arm outward. As a rule, these muscles are quite distinct, though they lie side by side; but in about one per cent of cases they are joined together by muscular fibers. This is the normal arrangement in apes and monkeys, the union of these two muscles aiding them greatly in twisting their bodies when hanging by their fore-limbs to the branches of trees. Again, in apes, the muscle forming the posterior fold of the arm-pit is always prolonged down to the prominence on the back of the elbow. In the long-armed apes this muscle is especially well developed, and serves to swing the whole arm rapidly and powerfully forward—a movement which is of the greatest importance for dexterously grasping remote branches while in the act of climbing. The same prolongation of this muscle is occasionally seen in man, though in a much less developed state, and serves to remind him of the arboreal habits of some of his not very remote ancestors.
In the gorilla, orang, and chimpanzee a muscle, called the elevator of the collar-bone (levator claviculoe), is always present; this goes from the upper neck-bones to the collar-bone. It is found in about three per cent of human subjects. Other muscles, occasionally found in man in a rudimentary and fragmentary condition, are ones going from the back of the head to the collar-bone or shoulder-blade; they are well developed in many of the carnivora and ruminants. I have seen them of large size in the lion, deer, etc.; in those animals they are much used in pulling forward the shoulder.
In about every other human subject is a small muscle going from a bony spur on the front of the haunch-bones to the muscles in the anterior wall of the abdomen. This is the rudiment of the great muscle in the kangaroo, opossum, and other marsupial animals, which supports the pouch where the immature young are carried, and the bony spur is the rudiment of a distinct bone, called the marsupial bone, which always exists in these animals, and gives attachment to the muscles which open and shut the pouch.
In man the short muscle of the foot which bends the toes is attached to the heel-bone, but occasionally the portion going to the fourth and fifth toes is separated from the portion going to the second and third toes, and is attached not to the heel-bone but to the tendon of the long flexor of the toes. In the gorilla only one slip of this short flexor arises from the long flexor of the toes, but in apes we have as a normal condition the arrangement I have endeavored to describe as that occasionally seen in man.
The brain of man is distinguished from that of the gorilla and the higher apes by having a greater relative size and being more complex. The different fissures are not so continuous, and are frequently bridged over by brain-matter. In the brains of criminals, the lower races of mankind, and idiots, according to Benedict, the fissures are very confluent in character, and in some the first frontal convolution is divided into two portions, as in apes. In animals lower in the scale than man, the little brain or cerebellum is more or less uncovered by the posterior lobes of the cerebrum or large brain. This uncovered condition of the cerebellum was well seen in an idiot's brain that I lately had the privilege of examining; the fissures were also of the confluent type; the whole brain only weighed sixteen ounces. The internal organs in man, although not subject to great variations, still are sometimes found abnormal. The liver may be divided into a number of lobes, as is seen in the gorilla. This is called a degraded liver. The spleen is often deeply notched and multiple, as in the case in some of the lower animals, and the uterus is occasionally double; an arrangement which is the normal one in the mare, raccoon, rabbit, and other animals. It is double in the human foetus up to the fourth month, and frequently a trace of this bifid condition is seen in adult life.
I could multiply, ad infinitum the variations in human anatomy which have their corresponding normal condition in the lower animals, but I think I have described a sufficient number of examples to show how common these animal resemblances are in man. On what theory can we account for their existence, except that they are reversions to some pre-existing and lower type? This is the only logical conclusion to which the study of morphology leads us, and "to take any other view," says Darwin, "is to admit that our own structure and that of all the animals around us is a mere snare laid to entrap our judgment."