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ART ANATOMY is a scientific explanation of the visible living form, or, in other words, it is the study of those parts of a living animal which influence its outward form or its expression. Besides the principal bones and muscles, this includes tendons, cartilages, sinew-sheaths, external veins and nerves, folds of skin, teeth, claws, beak, hoofs, horns, bristles, hair, feathers, &c.; and although it has been the custom of art anatomists to treat only of the first two mentioned, it will be seen that in many animals, the hair, and even the veins, are of far greater importance than many of the muscles or minor bones.
In depicting Birds also, a knowledge of the feathering is, generally speaking, of more value to the artist than familiarity with the separate muscles and bones. But a sound knowledge of the form of Mammals must be founded on an acquaintance with the bony skeleton and muscular system.
All Mammals are built on the same general plan; even the human form is but a slight variation of the same. All however are not equally good for study; some are poorly developed, some have the form obscured by wool, or by fat, some are too unwieldy or too minute, and others are not obtainable for study, through their rarity. But the Greyhound will be found well adapted to the requirements of the art anatomist, in his initial study of bones and muscles. It is superbly developed and the relation of its parts is admirably displayed through the fine skin and thin coat. It is neither too large nor too small, it is readily obtained almost anywhere, and when alive is perhaps easier to study than any other animal.
Natural sequence requires that the hairy coat be first treated or dissected. And it will be seen that this course is not entirely without justification in the importance of the subject.
Before proceeding further it will be well to give a word of warning regarding individual variation. In all departments of anatomy great allowance must be made for this, but especially in the muscles and the hair. It is probable, indeed, that the variations between a typical Dog and a typical Cat, Fox, Wolf, or Lion, &c., are not greater than those which will be found existing between different Dogs.CHAPTER 2
Plates I, II, III., IV.
IT is remarkable that the study of the Hair should have been so long and so entirely ignored. In all animals that bear it, it is of interest and value, in most it is of equal importance with the muscles, in many it is of much more consequence than these, ranking in value next to the bones as an element of form.
In the Horse, or the Greyhound, we see the hairy coat at a minimum, and yet in these the Hair has much to do with their appearance. In a Wolf the hair-masses are at least of equal importance with the muscles, and in a Grizzly, or Brown Bear, in its winter coat, the hair-masses and the bones give the clue to nearly all the visible form. In a Barye statue of a bear now before me it is impossible to detect the form of a single muscle, except on the arm. All the rest of the detail is worked out in hair-masses.
In very small Mammals the turn of the Hair is almost the only clue to the form, and the peculiar rounding and cracking on ham and on shoulder are the only indications of the complicated machinery of bones and muscles which lies far beneath.
The Greyhound has too scanty a covering to be a good subject for an initial study of Hair, so the Common Wolf (Canis lupus) will be used instead. This animal, practically the same throughout North America, Europe, and Northern Asia, is really a Wild Dog, with the best possible development of a Dog's powers. Its coat illustrates admirably all the essential features of furry covering, features which, though discoverable in the Greyhound, are in that animal so reduced as to be difficult of study.
The Hair of the Wolf and of most Mammals is of two kinds; a fine wool next the skin, and an outer covering of long, nearly straight hairs growing through this. The first retains the heat, and the second repels the rain. The first predominates on the lower and the second on the upper parts of the body.
The wool is also better than the hair for such parts as are very supple and change much in form, consequently we see a predominance of wool in those areas of loose or sliding skin under which the body has great play.
This may be due to the wearing off of the longer, brittler hair. The wool on these sliding areas is seen to crack open when the skin is extended. These peculiar tracts are so much more the result of arrangement than of actual change in the fur, that it is impossible to distinguish them in the animal after it is skinned or long dead; but no one can look at the living, moving creature and doubt their importance from a picturesque point of view.
These areas are limited in the Wolf, but in the Puma, or American Panther, we see them at a maximum, and the wonderful suppleness of this animal is aptly illustrated by the fact that its body is almost everywhere clad in this particular kind of covering.
The legs, shoulders, and face of the Wolf are covered by a variety of hair which is short, close and very hard. This is well calculated to give to the limbs and the senses perfect freedom of action, and at the same time is readily kept clear of mud, remnants of food, &c.
In general, the direction of the Hair is determined by two laws. First, the necessity of offering the least possible resistance to the air, and to grass, brushwood and other obstacles, while the animal is in motion. (This may be illustrated by the well-known fact that the hunter can readily drag, nose first, a dead deer which, heels first, he could scarcely move, for the obvious reason that it would be ' against the grain.') Second, the necessity for running off the rain, especially while the animal is lying at rest. The first law gives a backward, and the second a downward direction to the Hair.
But these rules are much broken by local requirements of more force, as will be seen in the Wolf. The first important exception is the curious radiation of the hair about the eye, with the object of clearing the way for the sight. This divergence is well shown in the American Buffalo; and among the feathered tribes, a notable parallel case is seen in the Owls. The hair of this radiation, meeting the counter-current of hair on the nose, produces the little ridge which is such a marked feature on the face of all hairy animals. (See Plates I., II., and III.)
On the side of the throat is a patch of reversed hair; it lies between the great thatch of the neck and the softer covering of the throat; it also covers the triangle between the upper and lower maxillary veins where they join the jugular.
This may be clearly seen in the Greyhound. (See Plate III.) After discussing with Dr. Caven the probable cause of this disturbance, we concluded that its history was briefly as follows :—
The early aquatic ancestor of living Mammals breathed by means of gills, which were gradually discarded as the creature became a land animal, and breathed by means of the elaborated air bladder, which we now call lungs. But with that conservatism so well known in organic bodies, the gill-cleft in the side of the neck persisted for long afterwards, and with it the accompanying circumvolution of the veins. This may be detected in the mammalian foetus, and when finally the old scar heals up, the disturbance in the surrounding hair is still to be seen, and in not a few cases the blood vessels preserve traces of the now useless circumvolutions.
There is a centre of divergence on the inside of each arm, as shown in Plate II., and the meeting of these radiations with the descending hair of the chest causes the ridges that are such a marked feature of the front view. (See Plate III.)
To explain these arrangements, I can suggest only the following theory.
The skin is formed from centres of pellification, just as the bone is formed from centres of ossification. At these centres the lowest layers are first formed, and in them the hair bulbs. The minute structures of the upper layer, having a tendency to push farther from the centre, would naturally give to the hair which traverses them, but which is fixed at its lower end, a tendency to diverge from the centre. The ridges of hair which appear as lines of convergence are the natural corollaries of the centres of divergence, they are the points where meet the two areas of independently formed skin, and may be styled Structural Scars.
The reversion of the hair on the back of the fore-legs may perhaps be explained in another way. The protection accorded by its position frees it from the operation of the first law; and the fact that the leg is horizontal when the animal is lying down would give the second law full force and reasonably account for its direction.
On the ischiatic bones we have radiating centres like those on the fore-arms, and on the inside of the hind-legs are reversed parts of the coat, as on the fore-legs. In Cows this area of reversed hair is called the 'milk-mirror,' as its extent is known to correspond with the amount of milk given by the animal. This has been explained on the ground that the direction of the Hair is determined by the main veins, and the amount of milk has, of course, a direct connection with the blood supply. This explanation, however, is not complete or satisfactory, even though one finds in other parts of the animal a remarkable coincidence between the presence of superficial blood vessels and of disturbance in the Hair-arrangement. These radiations are all remarkably clear in the Greyhound.
Plate I, shows how the Wolf illustrates the foregoing principles. The drawing is understood to be diagrammatic, as the disposition of fur therein shown, though more or less discernible in all Wolves, will but rarely be found as clear and sharp as in the drawing.
The chief masses are :—
The ruff, beginning before the ears, and passing over the back of the jaw and under the throat; this is much better developed in the Lynx and the Lion (Plate IV.).
The curious little cushion under each ear, a sort of central point or whorl of the several hair currents of the region.
The great thatch or mane of coarse hair; much developed in the Lion.
The soft woolly part under the throat meeting the mane; these two coincide nearly with the two parts of the Panniculus cervicis.
The mane along the shoulders, and the smaller crest on the top or dorsal edge of each scapula.
The patch of sliding fur behind each shoulder, and the corresponding patch on each flank, before the hind-leg, showing the great play of the limbs.
The two areas of reversed hair on the breast.
The fringe of reversed hair on the back of each fore-leg.
The great cushion of wool on each of the buttocks.
The tail, clad entirely in the sliding fur, except the slight thatch on the base above.
The ridges on the chest and on the belly, apparently structural scars as already defined.
The correlation of colour with this arrangement is striking.
The heavy thatches are much mixed with black hair; the thatch on the base of the tail is usually ended in a dark spot; the ruff on the cheeks is always paler in colour than the hair on the crown and neck; the sliding fur is always paler than the hair about it; while the close fur on the limbs and face is usually darker than elsewhere.
This arrangement both of form and of colour will be seen in all Dogs, Wolves, Jackals, and Foxes, and in a general sense is common to all the Carnivora; the leading features are to be found in all hairy quadrupeds, as well as in man himself. From this it will be seen that, excepting on the shoulder, the foreleg, and the hind-leg below the knee, these anatomical details of the hairy coat are in the Wolf, as well as in many other animals, of more importance than the anatomy of the muscles.CHAPTER 3
THE SKIN FOLDS.
IN many animals the Folds of Skin are of consequence. In the Rhinoceros for instance they are of much more importance than any of the individual muscles. In the present example, the Greyhound, we find several of these loose flaps, which though greatly reduced are still of value in picturesque effect.
First, the fold from the back of the arm to the ribs, that is, the roof of the armpit formed in part of the muscular fibres of the Flyshaker or Panniculus carnosus.
Second, the fold from the knee to the belly, forming the groin-flap or loose-flank.
Nearly all quadrupeds have this and the first well marked, and usually they are distinguished by a peculiar arrangement of hair.
Third, two loose folds on the throat arising one from each ramus of the lower jaw, uniting on the front of the neck. These are excessive in the Bloodhound and kindred races, as well as in Bulls and several Pachyderms. Their object apparently is to give free play to the neck and protection to the throat.CHAPTER 4
ALMOST the only Nerves we need to observe are those whose terminations are enlarged bosses on the skin in which are long specialised bristles, feelers or whiskers.
In the Dog we find these in four places on each side of the head, and in two places on the lower jaw:—
Ends of the Infraorbital Nerves, or Whisker-bed. This is the great nerve-bed at each side of the muzzle, out of which, in the Dog, grow the whiskers in four rows.
Malar, a small boss with four or five bristles.
Zygomatic, a smaller boss on the cheek above and behind the preceding—in the Greyhound with but few bristles.
Frontal, over the inner corner of each eye.
The Mental, on the lower surface of the end of the Lower Jaw; more or less provided with bristles.
Mylo-hyoid, a conspicuous boss with long bristles; it is situated on the interramal space beneath the lower jaw.
There is also a noticeable cord crossing the hough space; this is formed by a nerve and a vein lying side by side, and is strongly marked in the Greyhound. Some animals, as the Squirrels, have nerve-endings and bristles on the inner surface of the fore-legs.
These organs act as feelers of exquisite sensibility. They are usually most developed in the strictly nocturnal quadrupeds, and in those whose mode of life necessitates an absolutely silent course through the woods, when their eyes can avail but little. In such the bristles usually project the width of the body on each side of the head.
Excerpted from Art Anatomy of Animals by Ernest Thompson Seton. Copyright © 2014 Dover Publications, Inc.. Excerpted by permission of Dover Publications, Inc..
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