In man, the length of the metacarpus is contained about 5-1/2 times in that of the humerus; in the bear, it is contained 4 times; in the dog, 2-1/2 times; in the horse, 1-1/3 times only.
It is well known that the proportions vary according to race, and that what we have here given are but the general indications.
The Articulations of the Anterior Limbs
The knowledge of human arthrology which we presume the reader to have previously acquired makes it unnecessary for us to enter into numerous details regarding the configuration of the articular osseous surfaces and the disposition of the fibrous bands that retain them in position.
Accordingly, in the description which follows, and also in that of the articulations of the posterior limbs, we shall occupy ourselves but very briefly with the details above referred to, so as to devote ourselves especially to the indication of the movements--that is to say, of that which, while easily comprehended on recollection of former studies, presents the greatest interest from the artistic standpoint in these studies in comparative anatomy.
=The Scapulo-Humeral Articulation.=--The head of the humerus and the glenoid cavity of the scapula being in contact, the two bones are bound together by a rather loose articular capsule, which is strengthened by the muscles of this region which fulfil the function of active ligaments.
This articulation, so movable in every direction in the human species, is not so much so in quadrupeds; the arm in the latter, as also the shoulder, being kept in contact with the lateral region of the thorax by the numerous muscles which surround it.
Of the movements performed by the humerus, flexion and extension are the most extensive; those of abduction and adduction are much less so.
It is necessary, before proceeding further, to determine what the two princ.i.p.al movements which we have just mentioned really are, viz., flexion and extension.
We know that in man the displacements of the humerus which take place in the antero-posterior direction are known as movement or projection forwards, and movement or projection backwards, respectively. We do not say that the humerus is flexed or extended, because, in reality, on account of the position which the skeleton of the shoulder occupies, it is not able to flex or place itself on the line of prolongation of the scapula with which it articulates.
In quadrupeds it is not so. The humerus and the scapula are contained in almost the same vertical plane; and the bone of the arm can take, in relation to the latter, the positions characteristic of flexion and extension--that is, of approach to the scapula and removal from it.
What makes the meanings of these terms a little confusing is that, in human anatomy, some authors consider the backward movement of the humerus as extension, and the forward movement as flexion; in order to be able to compare these movements to those that the femur executes in relation to the pelvis.
Now, in our opinion, the indication of this correspondence is not absolutely necessary; since it ceases to be exact if we wished, from the point of view of the direction given to other segments of the skeleton, to establish the same relation between the elbow and the articulation of the knee.
It is therefore indispensable, when discussing quadrupeds, to discontinue these terms, in order the more readily to recognise that: in flexion the inferior extremity of the humerus is directed backwards; in extension, on the contrary, it is directed forwards. In the first case the humerus approaches the scapula; in the second, on the contrary, it moves away from it.
These movements, which take place during walking, are executed in the following manner: When one of the anterior limbs is at the end of that stage of progression which is called support (see p. 289, Displacements of the Limbs)--that is to say, during the time that the foot remains in contact with the ground, whilst the trunk is moving forward--the direction of this limb becomes more and more oblique downwards and backwards. At a certain moment the limb is raised from the ground, to be carried forwards, in order to be again pressed on the ground, and recommence a new resting stage. In these different phases the humerus is flexed. But at the moment that the limb, when carried forwards, is about to resume its contact with the ground it becomes directed obliquely downwards and forwards; then the humerus is in the position of extension.
During these movements of the humerus, there exists an essential factor--that is, the scapular balance. (It is the same as what occurs in man when he balances his arm in the antero-posterior plane.) When the humerus is flexed, the scapula moves in such a way that the superior portion projects forwards; when it is extended, the scapula, on the other hand, is inclined more backwards. But it is necessary to add that, during these displacements, the scapulo-humeral angle varies; it tends to close during the flexion of the humerus, and becomes more open during extension.
The movements and the relations of the humerus and the scapula are clearly represented in Figs. 41 and 42, reproduced from the chromophotographic studies of Professor Marey--studies relative to the a.n.a.lyses of the movements of the horse.[13] They show clearly the movements of flexion and extension of the humerus, also the balancing of the scapula which accompanies the movements.
[13] E. J. Marey, "a.n.a.lyses of the Movements of the Horse by the Chromophotograph" (_La Nature_, June 11, 1898).
[Ill.u.s.tration: FIG. 41.--FLEXION OF THE HUMERUS: RIGHT ANTERIOR LIMB OF THE HORSE, EXTERNAL SURFACE (AFTER A CHROMOPHOTOGRAPHIC STUDY BY PROFESSOR MAREY).]
[Ill.u.s.tration: FIG. 42.--EXTENSION OF THE HUMERUS: RIGHT ANTERIOR LIMB OF THE HORSE, EXTERNAL SURFACE (AFTER A CHROMOPHOTOGRAPHIC STUDY BY PROFESSOR MAREY).]
=The Articulation of the Elbow, or the Humero-ulnar Articulation.=--In this articulation, which is constructed in the form of a true hinge, the movements of flexion and extension alone are possible. In flexion, the forearm, directed forwards, is folded on the arm, with which, in certain circ.u.mstances, it comes in contact. For example, in a horse of mettle which leaps over an elevated obstacle, the animal forcibly raises his fore-limbs by flexing them. Flexion is produced to the same extent, and even more so, and for a longer period, in felides which crouch.
In extension, on the contrary, the forearm is carried backward. This movement being limited only by the contact of the tip of the olecranon with the bottom of the olecranon fossa of the humerus, the forearm is enabled, in this case, to move until it is in line with the arm. For example, during walking, when one of the anterior limbs, having reached the end of its resting stage, is considerably inclined downwards and backwards.
The apex of the olecranon process--that is to say, the point of the elbow--forms a marked prominence, more salient in flexion than in extension, as in the corresponding region of the human elbow.
=The Radio-ulnar Articulation.=--It is in the dog and the cat, in which the two bones of the forearm articulate by their extremities only, and remain separate in the rest of their extent, that the articulations call for special notice.
In the upper part, the radius rotates on itself; while below, it rotates around the ulna. It follows that the forearm, which in all quadrupeds is in a state of permanent p.r.o.nation, can, in carnivora, take the position of supination, or rather, of demi-supination. In fact, whatever be the mobility of the two bones of the forearm, the movement is not able to bring the palmar surface to the front, but only to direct it towards the median line.
=The Articulation of the Wrist.=--Here are found, as in man, three superimposed articulations: the radio-carpal, intercarpal, and carpo-metacarpal.
If we remember the movements which take place at the plane of these articulations in man, and take account of the fact that the mobility of the limbs is reduced just in proportion as they are simplified in structure so as to become organs of support only, we can easily comprehend that, in the horse and the ox, and, in a word, animals that have a canon bone, the movements of the wrist are little varied in character, while in carnivorae, on the other hand, they are relatively more so.
We will remember that in the ox and the horse the region of the wrist is called the _knee_.
In flexion, the hand is bent backwards; in extension it is carried forwards. These two movements take place especially in the radiocarpal and intercarpal articulations. In the first of these articulations, it is the superior row of the carpus which glides backwards and forwards on the corresponding articular surface of the forearm. In the second articulation, it is the second row which moves; gliding on the inferior articular surfaces of the row above it. This inferior row carries the metacarpus with it; for the carpo-metacarpal articulation is much less mobile than either of the other two.
In flexion, the articular surfaces are separated from one another in front; and the changes of form which result from this are noticeable on the anterior surface of the "knee." Moreover, at that moment this region contrasts markedly in its outlines with the parts above it and below it--that is to say, with the corresponding surfaces of the forearm and of the canon bone.
As for the lateral movements, by which the hand is inclined outwards and inwards in its movements at the wrist, they exist to an appreciable extent in the cat and the dog only; in order to understand this, it is enough to compare the shape of the articular surfaces of this region in carnivora and the horse, for example. In the latter, those surfaces are almost plane; in the cat, on the contrary, they are curved (inferior surface of the forearm, concave; superior border of the carpus, convex).
These latter, then, are, in form, similar to those which exist at the same level in the human being; this explains the possibility of a.n.a.logous movements of the whole hand--that is to say, of the movements of abduction and adduction.
=The Metacarpo-phalangeal Articulations.=--With regard to the mobility, it is in these articulations, as in those of the wrist--that is to say, although in all quadrupeds the first phalanges can be flexed and extended on the metacarpus, it is only in the cat and dog that lateral movement is possible. Indeed, in the horse, in which the princ.i.p.al metacarpal terminates inferiorly in two convex surfaces, which are separated by a crest; and where the whole articulates with a cavity on the superior extremity of the first phalanx; because of the hinging of these surfaces, there can only be movements of opening and closing of this articulation. The first phalanx is directed backwards during flexion and forwards during extension. In the dog and the cat the digits can be separated from each other, and also drawn together--that is to say, abducted and adducted; but, as in man, these movements can be made only when the first phalanges are in the state of extension. During flexion they are impossible, because of the tension of the lateral ligaments, which increases as the flexion is more p.r.o.nounced. This can be demonstrated, for example, in the cat, which, in order to separate the digits, opens the hand widely--that is to say, forcibly raises the first phalanges.
=The Interphalangeal Articulations.=--The phalanges are in contact with one another by surfaces, which, on one side, are of trochlear form, and, on the other, are moulded on these trochleae; accordingly, at the level of these articulations, the movements of flexion and extension only can take place.
In the felidae, the claws which the third phalanges bear cannot be utilized when the latter are in a state of extension, at which time, being forcibly raised, they are, in fact, placed on the outer sides of the phalanges, which are grooved to receive them. But when the animal wishes to use them, it flexes those third phalanges, of which the terminal extremity is then projected forward, and the claws are ready to fulfil their function. But at the same time it extends the first phalanges, to produce a certain tension of the flexors of the digits, and thus enable the latter to act with greater efficacy, with a minimum of contraction. We can demonstrate this action experimentally on ourselves. It is enough to carry the first phalanges forcibly into a state of extension; the third phalanges then become flexed, quite spontaneously, by the tension of the tendons of the flexors which are inserted into them.
At the same time, if we examine the felidae which we have taken as examples, when the first phalanges are in the state of extension, the digits will be found to be separable, as we have already indicated in connection with the metacarpo-phalangeal articulations, with the result that the claws are then able to lacerate a wider surface.
The extension of the ungual phalanx, which determines the retraction of the claw and stops its action, is the mechanical result of an elastic, fibrous apparatus which is attached to each of the third phalanges, and has its origin of the second.
THE POSTERIOR LIMBS[14]
[14] Examine Figs. 21, 33, 34, 38, 39, 49.
The posterior limbs are divided, as are the inferior limbs of the human being, of which they are the h.o.m.ologues, into four parts: pelvis, thigh, leg, and foot.
The Pelvis
The pelvis, which incompletely limits the abdominal cavity, inferiorly in the vertical position of the body and posteriorly in the normal att.i.tude of quadrupeds, is formed by the iliac bones and sacrum--the coccyx forming a prolongation of the latter. We have already described the two latter (pp. 10 and 11) in connection with the vertebral column, of which they form the inferior or posterior portion or segment, according to the att.i.tude of the individual.
=The Iliac Bone.=--The iliac or c.o.xal bone, is a paired or non-symmetrical bone, united below to its fellow of the opposite side, while it is separated from it above by the sacrum.
In all animals, as well as in man, the iliac bone, at the beginning of life, consists of three parts, which afterwards unite and fuse together and join at the middle of the bottom of a deep cavity which is situated on the outer aspect of the bone--the cotyloid cavity.
Of those three portions when examined in the human iliac bone, that above the cavity is the ilium; that on the inside is the pubis; and the last, the lower one, is the ischium. In quadrupeds, the iliac bone being, in its entirety, directed much more obliquely downwards and backwards, the relative position of these const.i.tuent parts is a little modified: the ilium is in front, the pubis is still internal, but in a more inferior position, and the ischium is behind the cotyloid cavity.
We notice this peculiarity of the development of the iliac bone because it is customary to continue to apply to the osseous regions which correspond to these parts the names by which they were known when independent bones.
[Ill.u.s.tration: FIG. 43.--THE LEFT ILIAC BONE OF THE HUMAN BEING: EXTERNAL SURFACE, PLACED IN THE POSITION WHICH IT WOULD OCCUPY IN THE SKELETON OF A QUADRUPED.
1, Cotyloid cavity; 2, ilium; 3, iliac crest; 4, anterior iliac crest; 5, posterior iliac spine; 6, pubis; 7, tuberosity of the ischium; 8, obturator foramen; 9, ischiadic spine.]
[Ill.u.s.tration: FIG. 44.--LEFT ILIAC BONE OF A QUADRUPED (HORSE): EXTERNAL SURFACE.
1, Cotyloid cavity; 2, ilium, external iliac fossa (directed upward in the horse); 3, iliac crest; 4, anterior iliac spine (directed inwards in the horse, it is the angle of the haunch); 5, posterior iliac spine (directed inwards in the horse; it is the angle of the haunch); 6, pubis; 7, tuberosity of the ischium; 8, obturator foramen; 9, ischiadic spine, or subcotyloid foramen.]