Chapter I: Osteology and Arthrology (2)

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Lastly, the ruminants (sheep, oxen, deer, etc.), and also the pig, ass, and horse, rest on the third phalanx only. In them not only is the metacarpus turned back, but also the two first phalanges. The wrist is very far removed from the ground. In these animals, the third phalanx is enclosed in a case of horn, a nail (the hoof), and because the support of the limb is on that nail, the name of unguligrades has been given them. Nevertheless, as the point of support is on the third phalanx, which is also known by the name of phalangette, we are of opinion that, in order to specify definitely, although they walk on their fingers, as do the digitigrades, the support is provided not by the whole extent of those appendages, they might receive the name of phalangettigrades.

It is necessary among the ruminants to make an exception of the camel and the llama, which are digitigrades.

Just in proportion as the hand is raised from the ground, as we have just seen in passing from the plantigrades to the digitigrades and unguligrades, the number of bones of that region diminishes, the bones of the forearm coalesce, and the ulna tends to disappear; the hand becomes less and less suitable for grasping, climbing, or digging, so as to form an organ exclusively adapted for walking and supporting the body.

Thus, the bear (plantigrade) has five digits, and the power of performing the movements of supination and pronation. Indeed, we know with what facility this animal is able to move his paws in every direction, and climb a tree by grasping it with his fore-limbs. It is well known, however, that no animal except the ape can perform the movements of rotation of the radius around the ulna with the same facility as man; and that none possesses the same degree of suppleness, extent, and variety of movements of the forearm and hand.

In the digitigrades there is one finger which is but slightly developed, and which is always removed from the ground--that is, the thumb: there is also a little less mobility of the radius around the ulna.

In the ungulates the limbs are simply required to perform the movements of walking, and form veritable columns of support, which become the more solid as they are less divided. The bones of the forearm are fused together; there is therefore no possibility of rotation of the radius around the ulna. The metacarpus is reduced to a single piece, which in the horse constitutes what is known as the _canon_. The number of digits becomes diminished, so that in ruminants there are not more than two, and in the horse but one. We should, however, add that, up to the present, we have taken into account only perfect digits, those that rest on the ground. We shall see further on that there exist supplementary digits, but that they are only slightly developed, and are represented in some cases by mere osseous spurs; it is this fact that has permitted us to ignore them in the general study which we have just made.

Because, as we have already said, the unguligrades have the inferior extremity of the digit encased in a horny sheath, which forms the hoof of the horse and the corresponding structures (_onglons_) in the ox, those animals have been placed in a special group, which is based on that peculiarity--that is, the group of ungulate mammals.

The plantigrades and digitigrades, of which the paws have their surfaces of support strengthened by an epidermic sole and fatty pads, have the free extremities of the third phalanges covered on their dorsal surface by nails or claws; hence they are named _unguiculate_ mammals.

The bat and birds have the bones of the forearm so arranged that the radius cannot rotate around the ulna. This is necessary in order that during flight, when the wing is being lowered, the radius and hand shall not be able to turn; for, if such rotation took place, each stroke of the wing would place it in a vertical position, which would occasion a loss of resistance incompatible with the effect to be obtained.

The Forearm

The skeleton of the forearm in quadrupeds is vertical in direction; consequently, it forms with the arm an angle open anteriorly; this is well seen on examining the lateral surface. If we examine it on its anterior surface, we find a slight obliquity directed downwards and inwards. In animals in which the bones of the forearm are separate--that is to say, susceptible of supination and pronation--we find a more close resemblance to those of the human skeleton. The ulna, the superior extremity of which always projects beyond that of the radius, has a shaft which gradually narrows from above downwards. Its inferior extremity is terminated by a round head in those animals in which the ulna is fully developed; in others, as it is atrophied, it ends in a thin, long process.

The ulna presents at its superior extremity a posterior process, the olecranon, which forms the point of the elbow. We find on the anterior surface of the same, another process, the coronoid.

It is necessary to dwell on the relations of these parts. In man the head of the radius is situated at the anterior part of the external surface of the superior extremity of the ulna (Fig. 25); indeed, the small sigmoid cavity with which the head articulates is situated on the outer side of the coronoid process, and this apophysis is placed in front. In the plantigrades and digitigrades the head of the radius is placed still more forward, so much so that it is situated almost in front of the superior extremity of the ulna (Fig. 26). In the unguligrades it is placed directly in front of this latter (Fig. 27).

Further, the displacement of the radius is made at the expense of the superior extremity of the neighbouring bone; the radius appears to appropriate more and more the parts which in man belong exclusively to the ulna--for example, the coronoid process. In the plantigrades and the digitigrades half of the process still belongs to the ulna and the remainder to the radius. In the ungulates--the horse, for example--the coronoid process belongs to the radius; the ulna, situated behind the latter, is correspondingly diminished in size.

1, Radius; 2, ulna; 3, olecranon process; 4, coronoid process.]

In brief, when we study this region of the skeleton in plantigrades, then in digitigrades, and finally in unguligrades, we find a kind of progressive absorption of one of the two bones (ulna) by the other (radius), which thus becomes the more developed.

It is easy to explain this partial disappearance of the ulna. When the forearm is capable of performing the movements of pronation and supination, the ulna is completely developed, for it is in its small sigmoid cavity that the head of the radius revolves, and it is around its inferior extremity, the head, that the corresponding extremity of the radius turns. But when the movements of rotation of the forearm do not exist, the inferior extremity of the ulna becomes functionally useless and disappears. As to its role in the movements of the region of the wrist, that is nil, for we may remember--we will observe it again when we come to treat of the articulations--that the hand articulates with the radius alone (radio-carpal articulation); this is the reason that, when the forearm possesses the fullest mobility, the hand follows the movements which that bone makes around the ulna.

1, Radius; 2, ulna; 3, olecranon process; 4, coronoid process.]

It is not so with the articulation at the elbow-joint; there it is the ulna, which, with the humerus, forms the essential parts (humero-ulnar articulation); its olecranon process limits the movement of extension of the forearm. It is for this reason that, even in those quadrupeds in which the ulna is atrophied, the olecranon process presents a relatively considerable degree of development.

1, Radius; 2, ulna; A, groove for the long abductor and short extensor muscles of the thumb; B, groove for the radial muscles; C, groove for the long extensor of the thumb; D, groove for the special extensor of the index finger and of the common extensor of the fingers; E, groove for the proper extensor of the little finger; F, groove for the posterior ulna.]

1, Radius; 2, ulna; A, groove for the long abductor and for the short extensor of the thumb; B, groove for the radials; D, groove for the long extensor of the thumb, the special extensor of the index-finger, and the common extensor of the fingers; E, groove for the special extensor of the little finger.]

We know that on the posterior surface of the inferior extremity of the bones of the human forearm are grooves in which pass the tendons of the posterior and external muscles which, belonging to this region, are directed for insertion towards the hand.

In animals, because of the movement of rotation of the radius, the surface of this bone, which is anterior, corresponds to the posterior surface of the same in man. (To possess a clear conception of this, it is necessary to remember that, in this latter, the bones of the forearm are always described as in the position of supination; they are thus represented in Fig. 28. The direction of the surfaces of the radius is the reverse of that in animals, since the latter have the radius always in a state of pronation.)

Consequently it is on the anterior surface of the bone that we find the grooves concerning which it is necessary to give some details. Regarding them in passing from the radius towards the ulna, those grooves give passage to the tendons of the muscles whose names occupy the columns on p. 43. The letters which are referred to each serve to define their order, and to facilitate reference to Figs. 28, 29, and 30.

1, Radius; A, groove for the long abductor and the short extensor of the thumb; B, groove for the radials; D, groove for the common extensor of the digits; E, groove for the special extensor of the little finger.]

We should mention that the groove E is situated, both in man and in the dog, at the level of the inferior radio-ulnar articulation; but that in the horse, as the ulna does not exist at that level, the groove is situated on the external surface of the inferior extremity of the radius. It is necessary to add that, in some horses, the ulna is, nevertheless, represented in this region by a tongue-like process of bone; and in such cases the groove is situated in front of this process, at the level of the line of coalescence, which there represents the articulation.

[11] The words printed in italics are the names used in veterinary
anatomy.

It is also useful to note, with reference to the groove F, in which passes, in man, the tendon of the posterior ulnar muscle, that, when the forearm is in pronation, the radius alone being displaced, we can only see this groove on the surface which looks backwards; and that it is then separated from the groove which contains the tendon of the special extensor of the little finger by an interval equal to the thickness of the head of the ulna.[12] When the forearm is supinated, the two grooves are found, on the other hand, one beside the other: and the tendons which they contain are very naturally in contact.

[12] Edouard Cuyer, 'Shape of the Region of the Wrist in Supination
and Pronation' (_Bulletin de la Societe d'Anthropologie_, Paris,
1888).

In birds the forearm is flexed on the arm, and the latter being directed downwards and backwards, the former is, consequently, directed upwards and forwards. Further, because of the position of the humerus, which, as we mentioned on p. 32, has its inferior extremity so turned that the surface which is anterior in man becomes external, the radius, instead of being outside the ulna, is placed above it. This latter is larger than the radius, but its olecranon process is very slightly developed.

The Hand

The hand in animals, as in man, is formed of three parts--the carpus, metacarpus, and fingers. In man, the forearm and the hand being described in the position of supination; the bones of the carpus are named in passing from the most external to the most internal--that is to say, from that which corresponds to the radial side of the forearm to that which corresponds to the ulnar side. In animals in which, as we know, but it is not unprofitable to repeat, the hand is in pronation, the radial side of the forearm being placed inside, we enumerate the carpal bones in counting the most internal as the first; this is the only method which permits us, in taking our point of departure from the human skeleton as our standard, to recognise the homologies of the bones of the carpal region.

These bones, eight in number, are arranged in two transverse rows, of which one, the first, is superior or antibrachial; the other, the second, is inferior or metacarpal. Each of these rows contains four bones. Considered in the order we have indicated above--that is to say, proceeding from the radial to the ulnar side--they are thus named: scaphoid, semilunar, cuneiform, and pisiform, in the first row; trapezium, trapezoid, os magnum, and unciform, in the second. The number of these bones is not the same in all animals on account of the coalescence or absence of some. In each row the bones are placed side by side, with the exception of the pisiform, which being placed on the palmar surface of the cuneiform, produces a small projection in man, but a very pronounced one in quadrupeds.

The pisiform is called the _hooked bone_ in some veterinary anatomies. If we consider the hook which it forms, we may recognise that the name is appropriate; but from the point of view of comparison with the human carpus, the name is unfortunate, for it creates confusion between the true pisiform (the fourth bone in the upper row), and the last bone in the lower row, which is the veritable unciform bone. We do not here seek for similarity of form, but homology of regions; and it is only by using the same names to denote the same things that we can succeed in determining such homology.

Taken as a whole, the bones of the carpus form a mass which, by its superior border, articulates with the bones of the forearm, and by its inferior border is in relation with the metacarpal region. Its dorsal surface (anterior in quadrupeds) is slightly convex; its palmar surface (posterior in quadrupeds) is excavated, and forms a groove in which pass the tendons of the flexors of the fingers. This last, in man, has the appearance of a gutter, because of the prominences caused by the projection of the internal and external bones beyond their fellows.

In quadrupeds the palmar groove is especially determined by the pisiform bone, of which we have just mentioned the great development.

The region occupied by the carpus, in the unguligrades, is known as the _knee_; it would have been more appropriately named had it been called the _wrist_.

The number of the metacarpal bones in mammals never exceeds five, but it often falls below it; the same is true for the digits. The first are generally equal in number to the latter; an exception is met with in ruminants, whose two metacarpals coalescing soon after birth, form but one bone; this, the _canon_ bone, articulates with two digits.

The number of metacarpals and digits diminishes in proportion as the limbs cease to be organs of prehension, and become more exclusively organs of support and locomotion.

The number of phalanges is two for the thumb and three for each of the other digits; except in the cetaceans, in which they are more numerous.

In the bat, the metacarpals and phalanges are very long, and form the skeleton of the wing; these phalanges are not furnished with nails; the thumb, which is very short, is alone provided with one (Fig. 8).

With regard to the relative dimensions of the bones of the metacarpus, it is necessary to remember that, in the human being, the second metacarpal is the longest; then, in the order of decrease, come the third, fourth, fifth, and first. In quadrupeds we shall also find differences in length (see the chapter relating to the anterior limbs in certain animals), but the order of decrease is not always that which we have just mentioned.

In man the articular surface, situated at the inferior extremity of each of the metacarpals, is rounded, and is called the head. This allows the first phalanx, which is in relation with that surface, to be displaced in every direction; indeed, this phalanx can not only be flexed and extended, but it can also be moved laterally; this latter movement allows of the fingers being separated and drawn together.

In quadrupeds which can only perform the movements of flexion and extension of the digits--for example, the horse--the inferior extremity of the metacarpal has not a rounded head of a regular outline; it is marked by a prominent median crest, directed from before backwards, so that the articular surfaces, which fit more exactly, form a sort of hinge which allows of backward and forward movements only, and permits no lateral displacement. In man, at the level of the inferior extremity of the first metacarpal, in the vicinity of the articulation of this bone with the first phalanx of the thumb, we find two sesamoid bones--small bones developed in the fibrous tissue which surrounds the articulation. We also meet with such structures, but more rarely, at the level of the corresponding articulation of the index and auricular digits; and, more rarely still, at those of the middle and ring fingers. In quadrupeds, these bones are normally developed, and we shall see afterwards that in some animals, as they reach a considerable size, they are able to influence the external outlines; we shall see this, for example, in the horse.

The hand, in birds, is directed obliquely downwards and backwards (Fig. 31). For the better understanding of its position in relation to the forearm, we should remember that this latter, as we have described (p. 44), directed obliquely upwards and forwards, has the radius placed above the ulna; the hand being oblique in the opposite direction and placed under the forearm is, by this arrangement, inclined towards the ulnar border of the latter.

1, Humerus; 2, radius; 3, ulna; 4, radial bone of the carpus; 5, ulnar bone of the carpus; 6, first metacarpal; 7, second metacarpal; 8, third metacarpal; 9, first digit, the homologue of the thumb; 10, first phalanx of the second digit; 11, second phalanx of the second digit; 12, third digit.]

For the rest, in order to be able to distinguish readily the corresponding parts in the hand of a bird and that of a man, we merely have to place the human forearm obliquely, in a direction upwards and forwards (Fig. 32), the radius being above; this position we can obtain by semi-pronation; then, to incline strongly the hand downwards and backwards, moving the ulnar border of the hand towards the ulna; the thumb is then anterior, the little finger posterior, and the palm of the hand is turned towards the trunk.

The carpus in birds is formed by two bones only, with which the skeleton of the forearm articulates. That which is in contact with the radius is called the _radial bone of the carpus_; and that with which the ulna articulates is named the _ulnar bone_.

The metacarpus is formed of three bones; the first, which is very short, is fused at its superior extremity with the adjoining one; this latter and the third, both longer than the first, but of unequal size, are fused at their extremities. The metacarpal, which articulates with the radial bone of the carpus, is larger than the one which is in line with the ulna. To the metacarpus succeed three digits, of which the central is the longest, and is formed of two phalanges; the other two are formed each by a small, stylet-shaped bone. The middle finger, situated on the prolongation of the metacarpal, which articulates with the radial bone of the carpus, has its first phalanx large and flattened transversely; this phalanx seems to have been formed by the union of two bones of unequal development; the second phalanx is styloid in form. As to the other two fingers, they are placed, one in front and the other behind; the first, which articulates with the short metacarpal, fused at its upper end with the principal bone of the metacarpus, in position represents the thumb. The other, which is the third finger, articulates with the inferior extremity of the thinnest bone of the metacarpus; it is sometimes closely united to the corresponding border of the first phalanx of the large--that is to say, of the median--digit.

The Anterior Limbs in Certain Animals

=Plantigrades=: =Bear= (Fig. 33).--The scapula of the bear approaches in shape to a trapezium, of which the angles have been rounded off. The anterior border (cervical) is strongly convex in the part next the glenoid cavity. The junction of the superior (spinal) and the cervical border forms almost a right angle, the summit of which corresponds to the origin of the spine. At its posterior angle there is a prominence, directed downwards, the surface of which is hollowed and is separated from the infraspinous fossa by a crest, so that at this level a third fossa is added to the infraspinous one. The neck of the scapula is but slightly marked. The acromion is prominent, and projects a little beyond the glenoid cavity.

The clavicle is rudimentary, but, as an example of the complete development of this bone in plantigrade quadrupeds, we may cite the marmoset.

The humerus is furnished at its superior extremity with a large tuberosity, wide, and situated in front of the head of the bone; the effect of this is that the bicipital groove is internal. As in man, the great tuberosity does not reach so high as the humeral head, but it approaches more nearly to that level. The deltoid impression is very extensive, and descends pretty far down on the body of the bone. The epitrochlea is prominent; the epicondyle is surmounted by a well-marked crest, curved and flexuous in outline.

1. Cranium; 2, face; 3, atlas; 4, axis; 5, seventh cervical vertebra; 6, first dorsal vertebra; 7, fourteenth and last dorsal vertebra; 8, lumbar vertebrae; 9, sacrum; 10, coccygeal vertebrae; 11, sternum; 12, ninth and last sternal rib; 13, costal cartilages; 14, acromion process; 15, third fossa on the external surface of the scapula; 16, great tuberosity of the humerus; 17, musculo-spiral groove; 18, epicondyle; 19, radius; 20, ulna; 21, olecranon process; 22, carpus; 23, pisiform; 24, metacarpus; 25, phalanges; 26, ilium, external fossa; 27, pubis; 28, tuberosity of the ischium; 29, obturator foramen; 30, great trochanter of the femur; 31, condyles of the femur; 32, patella, or knee-cap; 33, anterior tuberosity of the tibia; 34, fibula; 35, tarsus; 36, calcaneum, or heel-bone; 37, metatarsus; 38, phalanges.]

The articular surface, which is in contact with the radius, is not a regularly formed condyle; it is a little flattened on its anterior surface, and presents at this level a slight depression which corresponds to a small eminence on the anterior aspect of the superior extremity of the radius. The surface which articulates with the ulna, viewed on its anterior aspect, has the shape of a slightly-marked trochlea; except at the level of the internal lip, which, as in man, descends lower than the surface for articulation with the radius (condyle). Behind, the trochlea is more clearly defined.

The bear possesses a considerable power of rotation of the radius; the bones of the forearm are joined only at their extremities, while in the remainder of their extent they are widely separated. The ulna terminates below in a head and a styloid process; these articulate with the two last bones of the first row of the carpus--viz., the cuneiform and pisiform. The bones of the carpus are seven in number, the scaphoid and the semilunar being fused together.

The metacarpals, five in number, differ very little from one another in regard to length, though they increase in size from the first to the fifth; this may be demonstrated by looking at the palmar surface of the hand. It is the reverse of that which we find in man, for the fifth metacarpal is the thickest of all, and the first is the most slender.

At the level of each metacarpo-phalangeal articulation are two sesamoid bones.

The third digit is the longest. The terminal phalanges present two very different portions: one, the anterior, is curved and pointed; it serves to support the nail, whose shape it assumes; the other, posterior, forms a sort of sheath into which the base of the nail is received.

The inferior portion of the posterior surface of this latter part articulates with the second phalanx in the case of each of the last four digits, but with the first phalanx in the case of the thumb.

1, Cranium; 2, face; 3, atlas; 4, axis; 5, seventh cervical vertebra; 6, thirteenth and last dorsal vertebra; 7, lumbar vertebrae; 8, sacrum; 9, coccygeal vertebrae; 10, anterior extremity of the sternum; 11, xiphoid appendix; 12, ninth and last sternal rib; 13, costal cartilages; 14, spinal border of the scapula; 15, supraspinous fossa of the scapula; 16, infraspinous fossa of the scapula; 17, great tuberosity of the humerus; 18, deltoid impression; 19, musculo-spiral groove; 20, olecranon process; 21, radius; 22, carpus; 23, pisiform; 24, metacarpus; 25, sesamoid bones; 26, phalanges; 27, ilium, iliac crest; 28, pubis; 29, tuberosity of the ischium; 30, great trochanter of the femur; 31, patella, or knee-cap; 32, anterior tuberosity of the tibia; 33, fibula; 34, tarsus; 35, calcaneum, or heel-bone; 36, metatarsus; 37, sesamoid bones; 38, phalanges.]

1, Posterior or axillary border; 2, superior or spinal border; 3, anterior or cervical border; 4, spine of scapula; 5, coracoid process; AA', length of spinal border.]

1, Posterior or axillary border; 2, superior or spinal border; 3, anterior or cervical border; 4, spine of the scapula; 5, coracoid process; AA', length of the spinal border.]

=Digitigrades=: =Cat=, =Dog= (Fig. 34).--In these animals the anterior (cervical) border of the scapula is convex; the posterior (axillary) border is straight or slightly concave. The supraspinous and infraspinous fossae are of equal extent (Figs. 35 and 36). The neck is short. The spine of the scapula becomes more and more prominent towards its inferior extremity, where it ends in a twisted and inflexed portion, which represents the acromion process; this process terminates at the level of the glenoid cavity. The coracoid process is represented by a small tubercle, slightly curved inwards; this tubercle is situated above the glenoid cavity, at the inferior part of the cervical border.

In the dog, the posterior angle, formed by the junction of the axillary and the superior (spinal) borders, is obtuse; the spine rises perpendicularly from the surface of the bone. The width of the scapula, measured at the level of the spinal border (from A to A', Fig. 35), equals about half the length of the spine. We must, however, make an exception for the turnspit dog, in which the superior border equals three-fourths of that length. The scapula is, in this case, of a more compact type; it is broader, but shorter. In the cat, the anterior outline of the scapula, formed by the union of the cervical border and the corresponding half of the spinal, is more convex; the posterior angle is not obtuse, as in the dog. The spine is bent slightly downwards and backwards; before terminating in the acromion process it presents a triangular projection, the apex of which is directed downwards. The tubercle which represents the coracoid process is curved inwards more strongly than that of the dog, thus resembling more closely the appearance of this process in the human being.

All proportions considered, the scapula of the cat is broader than that of the dog; its width, measured along the length of its spinal border (from A to A', Fig. 36), equals three-fourths of the length of the spine.

The clavicle is rudimentary; it is, however, better developed in the cat than in the dog. The clavicle of the cat is represented by a small, elongated bone, curved in outline, the convexity being turned forward; it is united to the acromion and the sternum by ligamentous fibres; that of the dog is merely a scale-like osseous plate situated on the posterior surface of a muscle of this region (see Figs. 16 and 17).

The humerus is long and twisted in the shape of an S. The inferior articular surface has the form of a simple pulley, for the condyle is very slightly marked. The internal part of this articular surface descends lower than the external; this condition resembles that found in the human being, where the inner lip of the trochlea is lower than the condyle.

In the dog, the olecranon fossa communicates with the coronoid by an opening.

In the cat, there is a supra-epitrochlear canal (see Fig. 19), but no olecranon perforation.

The bones of the forearm articulate at their extremities. The body of the radius is united to the body of the ulna by a short, thick, interosseous ligament; the fibres of this ligament, though short, do not prevent the production of some movements at the articulations of the bones.

The radius so crosses the ulna that above, it is in front and external to the latter, while below, it is internal. This bone is flattened from front to back, and slightly convex anteriorly. Its superior extremity is formed, externally, of a portion which represents the head of the radius in man; internally, by another portion which represents half of the coronoid process of the ulna, which, in the human being, belongs exclusively to the latter (see p. 39, the encroachment of the radius on the ulna). This extremity is surrounded with a vertical articular surface which is placed in contact with a small cavity which is hollowed out on the ulna (the lesser sigmoid cavity); and presents at its superior aspect a surface which articulates with the inferior extremity of the humerus. The shaft of the bone has on its internal border rugosities analogous to the imprint of the pronator radii teres of the human skeleton; these rugosities, indeed, give insertion to a muscle of the same function, and bearing the same name. The inferior extremity, broader than the superior, is hollowed on its external aspect by a small cavity which receives the inferior extremity of the ulna; its inferior surface (concave) articulates with the carpus; its anterior surface (the homologue of the posterior surface of the corresponding extremity of the human radius) presents grooves which serve for the passage of the tendons of the muscles which pass from the forearm to the back of the hand. (For the names of the muscles whose tendons pass in these grooves, see Fig. 29.)

The ulna is furnished at its superior extremity with an olecranon process, which is more prominent than that of the human ulna; this process is compressed laterally, and its internal surface is hollowed; there we also find a great sigmoid cavity, and a coronoid process situated at the internal part of the anterior surface, a process which, as we have previously shown, it shares with the radius.

The shaft of the bone, prismatic and triangular, diminishes in thickness as it approaches the lower extremity, which articulates with the corresponding extremity of the radius. In the dog, the ulna terminates inferiorly in a blunt point, without enlargement, analogous to the head of the human ulna; in the cat, by a head which is prolonged into a styloid process, by which it articulates with a portion of the carpus.

The carpus consists of seven bones--three in the superior row and four in the inferior. In the superior row the scaphoid and semilunar bones are fused together. The pisiform is elongated and expanded at its two extremities; it forms a prominence which, directed backwards, projects beyond the level of the other bones of this region.

The metacarpal bones are five in number; they are enumerated from within outwards; they articulate with the carpus and with each other. The inferior extremity of each metacarpal bone presents the form of a condyle in front; and is divided behind so as to form two lateral condyles, which are separated by a median crest; on these posterior condyles are applied two small sesamoid bones. The metacarpal bone of the thumb is very short; the third and fourth are the longest. The metacarpus, as a whole, is directed vertically.

The phalanges are three in number for each finger, except the thumb, which has but two. The first phalanx, directed almost horizontally forwards, is the longest; the second is directed downwards and forwards; the third consists of two portions: a posterior part, which forms a sort of sheath into which the base of the nail is received; and an anterior, conical in form, and curved in crochet shape, which forms a support for the nail (Fig. 37).

The third and fourth digits are the longest; the second and fifth are of equal length; the thumb is the shortest; it does not touch the ground, and does not even reach the articulation of the metacarpal bone and first phalanx of the second finger.

In the cat, the metacarpal bone of the thumb, although shorter than any of the others, is quite as thick. The third digit is a little longer than either the second or fourth. In animals of this genus, the claws, in the condition of repose, are retracted, and removed from the ground; this prevents their being worn, and thus preserves their sharpness. At such times the third phalanx is received into a groove which is found on the external surface of the second phalanx. In the dog, the claws are not tractile.

1, Metacarpus; 2, sesamoid bones; 3, first phalanx; 4, second phalanx; 5, third phalanx; 6, gutter for the reception of the base of the nail; 7, prominent osseous crest formed to lodge in the concavity of the nail.]

1, Cranium; 2, face; 3, atlas; 4, axis; 5, seventh cervical vertebra; 6, first dorsal vertebra; 7, fourteenth and last dorsal vertebra; 8, lumbar vertebrae; 9, sacrum; 10, coccygeal vertebrae; 11, anterior extremity of the sternum; 12, xiphoid appendix; 13, seventh and last sternal rib; 14, costal cartilage; 15, cartilage of prolongation of the scapula; 16, great tuberosity of the humerus; 17, olecranon process; 18, radius; 19, ulna; 20, pisiform; 21, metacarpus; 22, phalanges of the two great toes; 23, phalanges of the external toe; 24, ilium; 25, pubis; 26, tuberosity of the ischium; 27, great trochanter; 28, knee-cap; 29, anterior tuberosity of the tibia; 30, fibula; 31, tarsus; 32, calcaneum; 33, metatarsus; 34, phalanges of the two great toes; 35, phalanges of the external toe.]

=Unguligrades=: =Pig= (Fig. 38).--The scapula is markedly narrowed in the region above the glenoid cavity. The spine is atrophied at both its extremities, so that at its inferior part we do not find the acromion process. In its middle portion the spine is prominent, and presents a triangular process which turns backwards, overlapping a part of the infraspinous fossa; this latter is much larger than the supraspinous. The spinal border is surmounted by the cartilage of prolongation, the superior margin of which is convex; this cartilage extends posteriorly beyond the posterior (axillary) border of the bone.

The small tuberosity of the superior extremity of the humerus is but slightly developed; the great tuberosity, on the contrary, is very large. The bicipital groove is situated internal to this. The deltoid impression is scarcely marked.

The forearm is short, directed obliquely downwards and inwards, thus forming with the hand an angle, of which the apex is directed inwards. The two bones of the forearm are strongly bound to one another by an interosseous ligament, which is formed of very short fibres. The radius appropriates, at its superior extremity, the coronoid process of the ulna. The latter is, notwithstanding, well developed in the rest of its extent; it has a flattened shaft which almost completely overlaps the posterior surface of the radius; its inferior extremity reaches to the carpus.

The carpus is formed of eight bones--four in the superior row, and four in the inferior. The third bone of the superior row (cuneiform) is more in contact with the ulna than with the radius.

There are but four metacarpal bones; there is no metacarpal of the thumb. The two median metacarpal bones are the longest; they are those which correspond to the digits which alone touch the ground. The internal digit and the external one are thin and short; they are functionless, as a rule, taking no part in supporting the limbs on the ground. Notwithstanding this, they are formed, as the other digits, of a number of phalanges, which give them the semblance of perfect digits. (We shall soon see that in certain animals there exist digits which, being incomplete with regard to the numbers of their constituent bones, more accurately merit the name of imperfect digits.)

The third phalanges are each enclosed in a horny hoof, to which the name of _onglon_ has been given.

We have already drawn attention to the smaller lateral digits, and noted the general fact that they do not come in contact with the ground. It is necessary to modify this statement by adding that under certain conditions they give a slight amount of support; for example, when the individual is the subject of excessive obesity, the limbs yield under the weight, and the nails of the lateral digits may touch the ground.

A similar fact may be noticed in pigs of ordinary bulk at the moment when, during walking, each of the fore-limbs commences to bear the weight--that is to say, when it is directed obliquely downwards and forwards; then all the digits are in contact with the ground.

=Unguligrades (Ungulates)=: =Sheep=, =Ox= (Fig. 39).--The scapula, which is of elongated form, is very narrow in the vicinity of the glenoid cavity. The spine, which becomes more and more salient towards its inferior part, terminates abruptly in a border, which, forming an acute angle with the crest, produces a projection which represents the acromion process--a very rudimentary acromion, for it does not reach the level of the glenoid cavity. The supraspinous fossa is much smaller than the infraspinous; it hardly equals one-third the extent of the latter. The anterior border, thin and convex in its superior portion, is concave in the rest of its extent; the posterior border is thick and slightly concave; the spinal border is surmounted by the cartilage of prolongation. In the ox the spine of the scapula, in its middle portion, is flexed a little backwards on the infraspinous fossa.

1, Cranium; 2, face; 3, atlas; 4, axis; 5, seventh cervical vertebra; 6, first dorsal vertebra; 7, thirteenth and last dorsal vertebra; 8, lumbar vertebrae; 9, sacrum; 10, coccygeal vertebrae; 11, sternum; 12, xiphoid appendix; 13, eighth and last sternal rib; 14, costal cartilages; 15, spine of scapula; 16, cartilage of prolongation of the scapula; 17, great tuberosity of the humerus; 18, musculo-spiral groove; 19, olecranon process; 20, radius; 21, carpus; 22, pisiform; 23, metacarpus; 24, rudimentary metacarpal; 25, sesamoid bones; 26, first phalanges; 27, second phalanges; 28, third phalanges; 29, anterior iliac spine; 30, pubis; 31, tuberosity of the ischium; 32, great trochanter; 33, supracondyloid fossa of the femur; 34, patella, or knee-cap; 35, anterior tuberosity of the tibia; 36, fibula; 37, coronoid tarsal bone; 38, tarsus; 39, calcaneum; 40, metatarsus; 41, rudimentary metatarsus; 42, sesamoid bones; 43, first phalanges; 44, second phalanges; 45, third phalanges.]

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 canal. 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 mass 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 principal 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 principal 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 principal 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 principal 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 principal 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 constitute 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 homologues 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 principal 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.

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, principal 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, principal 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 analogous 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.

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Artistic Anatomy of AnimalsChapter I: Osteology and Arthrology (2)

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