Artistic Anatomy of Animals Part 7

The great tuberosity of the humerus is highly developed; its summit, very prominent, is flexed over the bicipital groove; a prominence of the small tuberosity also bends over the groove, with the result that at this level the latter is converted into a sort of ca.n.a.l. At the inferior extremity the condyle, although not large, is recognisable; for it is separated from the trochlea by a depression in form of a groove. In contrast to the condition found in man, the condyle descends to a level a little below that of the internal lip of the trochlea. (For the arrangement of the epicondyle and the epitrochlea, see p. 30.) In the sheep, the deltoid impression is but slightly marked; in the ox, it is more evident.

The forearm is directed obliquely downwards and inwards, so as to form, with the hand, an angle of which the apex is internal; this angular outline of the _knee_ (wrist) is so characteristic of ruminants that the corresponding region of the horse, when salient inwards, receives the name of _ox-knee_. The radius bears the coronoid process, and the larger part of the articular surface which comes in contact with the inferior extremity of the humerus; the condyle and the trochlea articulate with the radius in front; while behind, the trochlea articulates with that part of the sigmoid cavity which belongs to the ulna. The posterior surface of the shaft of the radius is flattened; its anterior surface is slightly convex. The inferior extremity articulates with the carpus by a surface which is directed obliquely downwards and inwards. The shaft of the ulna is very slender, and fused in its middle third with the body of the radius; it terminates below, at the level of the external part of the inferior extremity of the radius, by a slightly expanded portion which, fused with this latter, forms the articular surface for the carpal bones.

In the ox the forearm is short; in the sheep it is proportionally longer.

The bones of the carpus are six in number--four in the upper row, and two in the lower; they form an irregular cuboid ma.s.s which contributes to the formation of the region known as _the knee_ in ruminants, as in the horse; we have already remarked that the name 'wrist' would be more accurate. The anterior surface in its foremost part is vertical, and is slightly convex from side to side. At its posterior and external part the pisiform bone forms a prominence.

The metacarpus consists of two bones only--one, well developed, which is known as the princ.i.p.al metacarpal, or the _canon_ bone (this is the name given to the region in the hoofed animals); and a rudimentary one, which is situated at the superior and external aspect of the preceding metacarpal. Sometimes there is found a third metacarpal at the internal aspect; but, when present, it is but very slightly developed.

The princ.i.p.al metacarpal consists of two metacarpals fused together; on this account the bone is longitudinally marked in the median line by a slight depression which marks the junction of the two bones of which it is formed. In some ruminants (certain species of chevrotains) the coalescence does not take place, and the two metacarpals remain separate.

The anterior surface of the princ.i.p.al metacarpal is convex transversely; its posterior surface is flattened. The superior extremity of this bone articulates by two facets with the two bones of the inferior row of the carpus; on the internal part of the anterior surface of this extremity is found a tubercle. The inferior extremity is divided into two parts by a fissure or notch; each part is articular, and consists of two separate condyles, which are separated from each other by an antero-posterior crest; on each side of this crest, and behind, are found two sesamoid bones. As for the external rudimentary metacarpal bone, it is nothing more than a small, short tongue of bone; which, in goats and sheep, is often absent.

The division of the inferior extremity of the princ.i.p.al metacarpal into two parts is correlated with the two perfect digits which give the foot of the ruminant its forked appearance. Each digit consists of three phalanges, which are directed obliquely downwards and forwards; further, these phalanges are inclined a little outwards from the axis of the limb, so that the two digits diverge from each other as they descend.

The first phalanx, which is the longest, articulates superiorly with the princ.i.p.al metacarpal; its inferior extremity terminates in a trochlea, and the lip of this, which is situated towards the axis of the limb, descends lower than that of the opposite side; this arrangement is correlated with the divergent direction of the digits. The second phalanx has its superior extremity moulded on the trochlea which terminates the extremity of the first; its inferior extremity is articular, and elongated from before backwards. On the posterior surface of this extremity is found a sesamoid bone.

With regard to the third phalanx, it presents the form of a triangular pyramid, and displays a postero-superior concave surface with which the second phalanx articulates; an anterior, convex surface, which terminates in a point on its anterior part; and an internal surface, which is flattened. The third phalanx of each digit is contained in a hoof (_onglon_).

There is also found in ruminants two imperfect rudimentary digits, which are represented by two small bones situated behind the articulation of the metacarpal and the digits which we have just been studying. These rudimentary digits are each enveloped in a layer of horn; they const.i.tute the _spurs_. The two digits of the ruminants represent the third and fourth fingers of the human hand; the two lateral digits, greatly atrophied, are the h.o.m.ologues of the second and fifth fingers; the thumb is not present.

It is the same as regards the metacarpal bones, which form, by their union, the princ.i.p.al metacarpal; the external represents the fourth metacarpal, and the internal the third. It is to the latter that the tubercle, of which we have already made mention, belongs; and with the signification of which, because it gives attachment to a muscle, we shall concern ourselves in the section on myology (see Radial Muscles).

=Unguligrades=: =Horse= (Fig. 40).--The scapula is narrow, compared with that of the animals we have just been considering. The anterior border is convex in its superior portion, and concave in its inferior; the posterior border is slightly hollowed out. The supraspinous fossa is less in extent than the infraspinous; but the difference is less than that between the same fossae in the ox and the sheep; in the ox, as we have already indicated, the proportion is one-third; in the horse, one-half. The spine, which disappears at the extremities, is rough and thick in its middle third, there forming a kind of tuberosity--_tuberosity of the spine_. Above and in front of the glenoid cavity is found a strong process consisting of a rugous base, and a summit which is directed inwards. This forms a kind of hook curved towards the inside; it represents the coracoid process. The scapula is surmounted by the cartilage of prolongation, of which the superior border, which is thin and curved, is parallel to the superior border of the prominence of the withers; the cartilage forms, consequently, the lateral surface of this region. The cartilage of prolongation undergoes ossification in old horses. The humerus is short; the bicipital groove, situated on the anterior surface of the superior extremity, separates the greater tuberosity from the lesser, and is divided into two parts by a median ridge; it is this portion of the humerus which forms the prominence known as the _point of the shoulder_, or _point of the arm_.

The deltoid impression well deserves the name of tuberosity which has been given to it, for it is very prominent; the musculo-spiral groove is very deep.

[Ill.u.s.tration: FIG. 40.--SKELETON OF THE HORSE: LEFT LATERAL SURFACE.

1, Cranium; 2, face; 3, atlas; 4, axis; 5, seventh cervical vertebra; 6, first dorsal vertebra; 7, eighteenth and last dorsal vertebra; 8, lumbar vertebrae; 9, sacrum; 10, coccygeal vertebrae; 11, sternum; 12, xiphoid appendix; 13, eighteenth and last sternal rib; 14, costal cartilage; 15, scapula; 16, cartilage of extension; 17, great tuberosity of the humerus; 18, deltoid crest; 19, olecranon process; 20, radius; 21, carpus; 22, pisiform; 23, princ.i.p.al metacarpal; 24, metacarpal, external rudimentary; 25, large sesamoids; 26, first phalanx; 27, second phalanx; 28, third phalanx; 29, ilium, showing external iliac fossa; 30, pubis; 31, tuberosity of the ischium; 32, great trochanter; 33, infratrochanteric crest, or third trochanter; 34, supracondyloid fossa of the femur; 35, knee-cap; 36, anterior tuberosity of the tibia; 37, the fibula; 38, tarsus astragalus; 39, calcaneum; 40, princ.i.p.al metatarsal; 41, rudimentary external metatarsal; 42, large sesamoids; 43, first phalanx; 44, second phalanx; 45, third phalanx.

_To face p. 64._]

At the inferior extremity, the trochlea is large; the portion corresponding to the condyle of the humerus in man is, in proportion to the latter, of small extent. The olecranon fossa is deep. The epicondyle and the epitrochlea are somewhat different from those of the human bone.

In the latter, the epitrochlea is salient towards the inner side, causing an increased transverse diameter of the inferior extremity of the humerus. In the horse--it is the same in ruminants--this tuberosity projects backwards, folds on itself in forming the internal boundary of the olecranon cavity, and exceeds in diameter, in the antero-posterior direction, the prominence of the epicondyle, which presents a nearly similar arrangement. This latter has, however, a part which, projecting externally, is situated at the inferior part of a crest, that forms the posterior boundary of the musculo-spiral groove. The result is that, contrary to the condition found in the human being, the epicondyle is more prominent transversely than the epitrochlea, but this latter is more salient on the posterior aspect. The epitrochlea and the epicondyle offer a larger surface for the origin of muscles of the forearm than the same prominences in the human bone do for the a.n.a.logous muscles of the same region.

Some veterinary anatomists have given to the inferior and external articular surface of the humerus the name of trochlea; and to the internal one, that of condyle. On this account they designate the external prominence as the epitrochlea, and the internal one as the epicondyle. In addition to the fact that this point of view is not legitimate, it produces inevitable confusion when comparing the parts with those of the human humerus, and this confusion exists, not alone in describing the bone, but also in the description of the muscular attachments, and in the comparison of the muscles of the forearm of quadrupeds with the corresponding muscles in the human species.

The radius is placed in front of the ulna; its body, slightly convex forwards, has the anterior surface convex transversely, and the posterior surface plane in the same direction. It is to the external part of this latter that the ulna is applied, which is completely fused with the radius.

The superior extremity of the radius is a little larger than the inferior. Its superior aspect, concavo-convex, moulded on the inferior articular surface of the humerus, presents internally two cavities, which receive the lips of the trochlea, and, externally, another, smaller, cavity, which receives the condyle. The radius articulates with the trochlea and the condyle, having appropriated a portion of the ulna, as is proved by the presence of the coronoid process, which belongs to the former. This superior extremity presents, internally, a tuberosity into which the biceps is inserted; this is the bicipital tuberosity; and on the other side is another tuberosity, which is a little more prominent than the preceding.

The inferior extremity, which is flattened from before backwards, is furrowed on its anterior surface by grooves for the pa.s.sage of muscles (the names of the muscles whose tendons pa.s.s in these grooves have already been given on p. 43). It articulates at the lower end with the superior row of the carpus, and it terminates laterally in tuberosities: one, external, on which is found a groove for the tendon of the lateral extensor of the phalanges, the h.o.m.ologue of the special extensor of the little finger; the other, internal, is a little more prominent than the one we have just described. These tuberosities are visible under the skin which covers the superior and lateral parts of the region known as the _knee_; but which, we again repeat, is no other than the wrist.

The ulna has a triangular shaft, situated at the posterior surface of the radius, with which it is fused. It disappears completely at the level of the inferior third of the forearm. Occasionally, in some horses, the ulna is abnormally long, in the form of a slender tongue of bone; and extends to the neighbourhood of the external tuberosity of the inferior extremity of the radius (see Fig. 79, p. 196). Its superior extremity is chiefly represented by the olecranon process, which is voluminous in bulk, and forms the projection known as the point of the elbow. This process is flattened laterally; its internal surface is excavated; the anterior surface, which is concave, forms a part of the great sigmoid cavity; the remainder of the cavity is formed by the radius.

In the a.s.s, the ulna is a little longer than in the horse--that is to say, it descends lower; and the radius is a little more convex anteriorly.

The carpal bones are seven in number--four in the superior row, and three in the inferior. The trapezium is wanting in the latter.

Sometimes, however, in certain varieties of horses the trapezium is developed, but then it is no more than a very small osseous nodule. The pisiform bone, situated at the external part of the first row of bone, is prominent posteriorly. It is of rounder form and flattened from without inwards. It articulates with the trapezium and the radius. It presents, on its external surface, a groove for the pa.s.sage of the tendon of the posterior ulnar muscle, which is named by veterinary anatomists the _external flexor of the metacarpus_.

The carpus, as a whole, is of an irregularly cuboid shape; its anterior surface, slightly convex from side to side, forms the skeleton of the region of the _knee_ (wrist). The metacarpus is formed of three bones: the princ.i.p.al metacarpal and the two rudimentary ones.

The princ.i.p.al metacarpal, which forms the region of the _canon_, is directed vertically; its anterior surface is slightly convex transversely. This surface is covered by a number of tendons, which slightly alter its appearance; so that it is the princ.i.p.al base of this part of the fore-limb. Its posterior surface is flattened. The superior extremity of this metacarpal presents plane surfaces, variously inclined, with which the bones of the inferior row of the carpus articulate. On the anterior surface, and a little to the inner side, is found a tuberosity, which is destined for the insertion of _the anterior extensor of the metacarpus_, the h.o.m.ologue of the radial muscles. The inferior extremity is formed by two condyles, an internal and an external; between which is found a median crest.

This extremity, the superior extremity of the first phalanx, which articulates with it, together with two sesamoid bones--the great sesamoids--which are situated on its posterior surface, collectively form the region which from its rounded outlines is called the _ball_.

With regard to the rudimentary metacarpals, external and internal, to which some authors give the name of _fibulae_, they are applied to the sides of the posterior surface of the princ.i.p.al metacarpal. They are elongated bones, of which the superior extremity, which is a little thickened, is called the _head_; the lateral bones of the second row of the carpus partly rest on the heads of these. They become more slender as they descend, and terminate opposite the inferior fourth of the princ.i.p.al metacarpal. Each ends in a slight swelling, to which the name _b.u.t.ton_ has been given. The internal one is the better developed.

The rudimentary metacarpals are vestiges of atrophied digits, as will be explained further on.

The single finger of the horse consists of three phalanges. The first phalanx, which is directed obliquely downwards and forwards, corresponds to the constricted region situated below the 'ball,' and known as the _pastern_. It is flattened from before backwards; its anterior surface is convex transversely, while the posterior surface is plane. Its superior extremity is moulded on the inferior extremity of the princ.i.p.al metacarpal, and its inferior extremity, which is smaller, presents a trochlea with which the second phalanx articulates. This is also directed downwards and forwards, and is shorter. It corresponds to the region which, situated between the pastern and the hoof, is known as the _cornet_.

The third phalanx, situated entirely within the hoof, has the same direction as the first and second. It is large and broad, and presents three surfaces separated by well-marked angular borders (see Fig. 96).

The anterior surface is oblique downwards and forwards; it is convex transversely. The inferior surface is slightly hollowed, and is in relation with the sole, or plantar surface of the hoof.

The superior surface, which is articular, is divided by a median ridge into two lateral cavities, which correspond to the trochlea on the inferior surface of the lower extremity of the second phalanx. The inferior border corresponds in shape with the hoof. The superior border presents in its median part a projection, _the pyramidal eminence_, which prolongs at this level the anterior surface of the bone. Finally, the posterior border, which is concave, is in contact with a sesamoid bone, _the lesser sesamoid_, which increases the superior articular surface behind, and is also in contact with the second phalanx.

As we have just seen, the horse possesses but one digit. In the ancestors of the animal--that is, in the prehistoric species which are now extinct (_orohippus_, _miohippus_, _protohippus_, or _hipparion_)--the number of digits was larger; this fact conclusively proves that the rudimentary metacarpals of the existing horse are vestiges of digits which have disappeared through want of use. In the first of those ancestors--orohippus--there were four digits; all save the first, the thumb, being then developed. In the others of the series there existed but three digits. It must, however, be noted that in those animals it is always the digit which corresponds to the middle finger of the pentedactyl hand that is longest. In other less ancient species the lateral fingers are reduced to the condition of mere splints of bone. It follows from what has been said that the digit which persists in the equine species should be considered as the third finger, and that the rudimentary metacarpals represent lateral digits considerably atrophied.

This disappearance of the lateral digits cannot excite surprise when we consider the functions of the organs. Becoming useless, they must undergo gradual atrophy from want of use.

There undoubtedly is, in this former existence of supplementary digits in the horse, something a.n.a.logous to what we still find in the pig; where the two princ.i.p.al digits are accompanied by two shorter ones, which very probably, from their infrequent use, are destined to disappear in a more or less distant future.

Proportions of the Arm, the Forearm, and the Metacarpus

As a supplement to the study of the anterior limbs which we have just finished, it appears necessary to give some indications of the relative proportions of certain of the segments which form these limbs in the plantigrades, the digitigrades, and the ungulates.

First, we would remark that, in following this order of cla.s.sification, the scapula becomes less and less narrow, and a.s.sumes a form more and more elongated. In order to convince ourselves of this, it will be sufficient to study the bone first in man, then in the bear, the cat, dog, ox, and finally in the horse.

As to the proportions of length, which are those we should chiefly study, we shall commence with the comparison of the forearm and arm--that is to say, the radius and the humerus. The radius is found to be longer in proportion to the humerus, as the number of digits is smaller, and the hand loses more and more the functions of an organ of prehension. In man, the radius is shorter than the humerus; in the horse, on the contrary, it is longer.

To give an idea of this proportion, we shall employ what is known as the antibrachial index. This index gives the relation which exists between the length of the forearm and that of the humerus; the length of this latter, whatever may be the actual measurement, is represented by a fixed figure, the number 100. A very simple arithmetical operation gives the proportion--

forearm 100 -------------, the quotient obtained furnishes the index.

humerus

The index is less than 100 if the forearm is shorter than the bone of the arm. The index is more than 100 if, on the contrary, the forearm is longer.

In man, the radius is shorter than the humerus; indeed, in adult individuals of the white race the average index is 74.

In the bear, the length of the radius approaches closely to that of the humerus; the index is about 90. In the skeleton of a bear in the anatomical museum of the ecole des Beaux-Arts, the humerus is 33 centimetres in length, and the radius 30 centimetres.

In the cat, the radius is very little shorter than the humerus. In the dog they are equal. The antibrachial index of the latter is, accordingly, 100.

In the horse, the radius is longer than the humerus; the index is therefore above 100. Thus, in the skeleton of the horse which we have in the museum of the ecole des Beaux-Arts, the index is 113--length of humerus, 29 centimetres; length of radius, 33 centimetres. In other skeletons which we have measured we found: in one, 108--humerus, 34 centimetres; radius, 37 centimetres; in another, 116--humerus, 25 centimetres; radius, 29 centimetres.

The metacarpal bone undergoes, relatively to the humerus, a proportional elongation, a.n.a.logous to that of the forearm.