Artistic Anatomy of Animals Part 5

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 p.r.o.nation 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.

[Ill.u.s.tration: FIG. 27.--SUPERIOR EXTREMITY OF THE BONES OF THE FOREARM OF THE HORSE: LEFT LIMB, SUPERIOR SURFACE.

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.

[Ill.u.s.tration: FIG. 28.--INFERIOR EXTREMITY OF THE BONES OF THE FOREARM OF A MAN: LEFT SIDE, POSTERIOR SURFACE, POSITION OF SUPINATION.

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.]

[Ill.u.s.tration: FIG. 29.--INFERIOR EXTREMITY OF THE BONES OF THE FOREARM OF THE DOG: LEFT SIDE, ANTERIOR SURFACE, NORMAL POSITION--THAT IS, THE POSITION OF p.r.o.nATION.

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 pa.s.s 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 p.r.o.nation.)

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 pa.s.sing from the radius towards the ulna, those grooves give pa.s.sage 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.

[Ill.u.s.tration: FIG. 30.--INFERIOR EXTREMITY OF THE BONE OF THE FOREARM OF THE HORSE: LEFT SIDE, ANTERIOR SURFACE.

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.

-----------------------+-----------------------+----------------------- MAN. | DOG. | HORSE.

-----------------------+-----------------------+----------------------- A. Long abductor of the|A. Long abductor of the|A. _Oblique extensor of of the thumb, and | thumb and short | the metacarpus_, the short extensor of | extensor of the | h.o.m.ologue of the the thumb. | thumb united as one | long abductor of the | muscle, _the oblique| thumb and the short | extensor of the | extensor of the | metacarpus_.[11] | thumb, united as one | | muscle.

| | B. First and second |B. The two radials |B. The radials external radials | blended superiorly, | represented by a (_extensor carpi | distinct inferiorly;| single muscle, _the radialis longior_ | this is _the | anterior extensor of and _brevior_). | anterior extensor of| the metacarpus_.

| the metacarpus_. | | | C. Long extensor of the|C. Long extensor of the|C. The long extensor of thumb. | thumb and special | the thumb and the | extensor of the | special extensor of | index finger united | the index are | superiorly. These | absent.

| muscles pa.s.s in the | | following groove. | | | D. Special extensor of |D. Common extensor of |D. _Anterior extensor the index finger and| the digits and the | of the phalanges_, the common extensor | two preceding | the h.o.m.ologue of the of the fingers. | muscles. | common extensor of | | the digits.

| | E. Special extensor of |E. _Extensor of the |E. Lateral extensor of the little finger. | third, fourth, and | the phalanges, the | fifth digits_, or | h.o.m.ologue of the | _the lateral | special extensor of | extensor of the | the little finger.

| digits_, the | | h.o.m.ologue of the | | special extensor of | | the little finger. | | | F. The posterior ulnar |There does not exist on the forearm a groove (_extensor carpi |for the posterior ulnar muscle, or _external ulnaris_). |flexor of the metacarpus_.

[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 pa.s.ses, in man, the tendon of the posterior ulnar muscle, that, when the forearm is in p.r.o.nation, 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 p.r.o.nation' (_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 pa.s.sing 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 p.r.o.nation, 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 h.o.m.ologies 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 p.r.o.nounced 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 h.o.m.ology of regions; and it is only by using the same names to denote the same things that we can succeed in determining such h.o.m.ology.

Taken as a whole, the bones of the carpus form a ma.s.s 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 pa.s.s 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.

[Ill.u.s.tration: FIG. 31.--SKELETON OF THE SUPERIOR LIMB OF A BIRD (VULTURE): LEFT SIDE, EXTERNAL SURFACE.

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 h.o.m.ologue 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-p.r.o.nation; 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_.

[Ill.u.s.tration: FIG. 32.--SUPERIOR LIMB OF THE HUMAN BEING, THE DIFFERENT SEGMENTS BEING PLACED IN THE ATt.i.tUDE WHICH THE CORRESPONDING PARTS OCCUPY IN BIRDS: LEFT SIDE, EXTERNAL SURFACE.]

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 princ.i.p.al 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