Artistic Anatomy of Animals Part 4

In the human skeleton, the internal lip of the trochlea descends lower than the external; and also lower than the condyle. In the bear, the cat, and the dog, it is the same. In the ox and the sheep, the condyle is lower than the trochlea, but only very little lower. In the horse the arrangement is still the same, but a little more accentuated.

On the lateral parts of this extremity we find: internally, a prominence, the epitrochlea; and, externally, another, the epicondyle.

It is from this latter that the crest arises, which, pa.s.sing upwards, forms the posterior limit of the groove of torsion.

The two prominences, which we have just described from a general point of view, present special arrangements which it is necessary to point out. When we examine the form of the outline of the inferior extremity of the humerus in man, the bear, the cat, the dog, the ox, and the horse, we find in following this order that the extremity tends to become narrow transversely, and that the epicondyle and the epitrochlea are less and less prominent on the external and internal aspects respectively. These two processes, indeed, project backwards; the epitrochlea always remaining more developed than the epicondyle. Because of this projection backwards, the cavity situated on the posterior surface of the inferior extremity, the olecranon fossa, is very deep, more so than in the humerus of man. Its borders being thus formed by the two processes, are very prominent. In front we find the coronoid fossa, which is less deep than that of which we have just spoken.

There exists in some mammals an osseous ca.n.a.l, situated above the epitrochlea, and known as the _supratrochlear ca.n.a.l_ (Fig. 19). It is bounded by a plate of bone which at its middle portion is detached from the shaft of the humerus, and blends with the latter at both its extremities. The brachial artery and median nerve pa.s.s through the foramen.

[Ill.u.s.tration: FIG. 19.--INFERIOR EXTREMITY OF THE LEFT HUMERUS OF A FELIDE (LION).

1, Epitrochlea; 2, supra-epitrochlear foramen.]

[Ill.u.s.tration: FIG. 20.--INFERIOR EXTREMITY OF THE LEFT HUMAN HUMERUS, SHOWING THE PRESENCE OF A SUPRATROCHLEAR PROCESS.

1, Epitrochlea; 2, supra-epitrochlear process.]

A similar condition is sometimes found, as an abnormality, in man, which presents itself under the following aspect (Fig.

20): an osseous prominence more or less long, in the shape of a crochet-needle--supra-epitrochlear process--situated 5 or 6 centimetres above the epitrochlea; the summit of this process gives attachment to a fibrous band, which is inserted by its other end into the epitrochlea and the internal intermuscular aponeurosis. The fibro-osseous ring thus formed gives pa.s.sage to the brachial artery and the median nerve, or in case of a premature division of this artery to the ulnar branch of the same.[9]

[9] For further details of this anomaly, see Testut, 'The Epitrochlear Process in Man' (_International Journal of Anatomy and Physiology_, 1889); A. Nicolas, 'New Studies on the Supratrochlear Process in Man' (_Review of Biology of the North of France_, t.

iii., 1890-1891).

There is also found in some mammals a perforation of the thin plate of bone which, in others, separates the olecranon fossa from the coronoid.

This perforation is sometimes found as an abnormality in the human humerus.

As does the sternum and the skeleton of the shoulder, the humerus of birds presents differences correlated to the functions which the thoracic limbs are destined to fulfil. Lying on the side of the thorax, directed obliquely downwards and backwards (Fig. 21), it is proportionately longer in individuals of powerful flight than in those which fly less or not at all. In the vulture it projects beyond the posterior part of the pelvis; in the c.o.c.k it does not even reach the anterior border of the same. To these differences in length are added differences in volume and in the development of the processes which serve for muscular attachment, which are more considerable in birds of powerful flight.

The humerus is so placed that the radial border, external in man and quadrupeds, looks upwards, with the result that the surface of the bone of the arm, which in these latter is anterior, in the former looks outwards. The humeral head, which is turned forwards and a little inwards, is convex and elongated in the vertical direction. Behind and above this head is found a crest for the insertion of muscles. It is the same for the region below, where there is a tuberosity whose inferior surface presents a pretty large opening which looks inwards to a fossa from the floor of which a number of minute openings communicate with the interior of the bone. This is the pneumatic foramen of the humerus.

It is of interest to remember in connection with this subject that in birds, in keeping with the conditions of flight, every system of organs is adapted to diminish the weight of the body. We particularly draw attention to the osseous framework, the structure of which is such that the weight of the animal is greatly lessened. This condition is secured by the pneumaticity. The bone consists of a cover of compact tissue, which, instead of enclosing marrow, is hollowed out by cavities which contain air, and communicate with special pouches, the air-sacs, which are appendages of the lungs.[10]

[10] The presence of air in the bones does not seem to be always a.s.sociated with the power of flight; as a matter of fact, we find air s.p.a.ces in the bones of some birds which do not fly (E. J.

Marey, 'The Flight of Birds,' Paris, 1890, p. 51).

[Ill.u.s.tration: FIG. 21.--SKELETON OF A BIRD (VULTURE): LEFT SURFACE.

1, Cranium; 2, face; 3, cervical vertebrae; 4, spinous processes of the dorsal vertebrae; 5, coccygeal vertebrae; 6, sternum; 7, keel; 8, superior ribs; 9, inferior ribs; 10, clavicle; 11, coracoid bone (for the details of the skeleton of the shoulder, see Fig. 18); 12, humerus; 13, radius; 14, ulna; 15, carpus; 16, hand (for details of the skeleton of this region, see Fig. 31); 17, ilium; 18, ischium; 19, pubis (for the details of the pelvis, see Fig. 46); 20, femur; 21, tibia; 22, fibula; 23, osseous nodule, which some anatomists think represents the calcaneum; it is the sole vestige of the tarsus; 24, metatarsus; 25, foot; 26, first toe (for the details of the skeleton of the foot, see Fig. 48).]

The antibrachial extremity of the humerus is flattened from without inwards. It terminates in two articular surfaces, which articulate with the radius and ulna.

The olecranon process of the ulna being slightly developed, it follows that the olecranon fossa is not large; neither is the coronoid.

General View of the Form of the Forearm and Hand

We now proceed to the study of the two regions of the fore-limbs which present the greatest variety in regard to the number of bones and also in regard to form and proportions. These two regions are the forearm and the hand.

It is first of all necessary to say that in man, when the fore-limb hangs beside the body, and the dorsum of the hand looks backwards, the two bones of the forearm are parallel, and that this position is known by the name of _supination_. It is also necessary to remember that there is another att.i.tude, in which the radius, crossing the ulna, and carrying the hand with it, displaces the latter in such a way that the palmar surface looks backwards. This second position is known as _p.r.o.nation_.

Let us now suppose that a man wishes to walk in the att.i.tude of a quadruped. It will be necessary, in order that his upper limbs, being for the moment anterior ones, may act as members of support, to place the forearm in p.r.o.nation, in order that, as is more normal, the hands may rest on the ground by their palmar surfaces. In this position the radius, being rotated on its own axis at its upper extremity and around the ulna in the rest of its extent, shall have its inferior extremity situated on the inner side of the corresponding extremity of the latter.

Such is the situation of the bones of the forearm and the att.i.tude of the hand in quadrupeds. In short, quadrupeds have their anterior members in the position of p.r.o.nation.

The individual whom we have just supposed placed in the att.i.tude of a quadruped would be able to maintain this position by pressing on the ground more or less extensive portions of his hands; the whole palm of the hand may be applied to the ground (Fig. 22); or the fingers only--that is to say, the phalanges (Fig. 23); or the extremities of the fingers only--that is to say, the third phalanges (Fig. 24). This last position, which is certainly difficult to maintain, should here be regarded rather as theoretical.

We shall meet with each of these modes of support in certain groups of animals. Thus, the bear, badger, and the majority of rodents, have the paws applied to the ground by the whole extent of the palmar surface of the hand, from the wrist to the tips of the fingers. They are therefore called plantigrade, from the a.n.a.logy, in this case, of the palm of the hand to the plantar surface, or sole of the foot.

[Ill.u.s.tration: FIG. 22.--THE HUMAN HAND RESTING FOR ITS WHOLE EXTENT ON ITS PALMAR SURFACE: LEFT SIDE, EXTERNAL SURFACE.]

In others, such as the lion, tiger, panther, cat, wolf, and fox, the support is made no longer on the whole extent of the palmar surface, but on the corresponding surface of the fingers only--the metacarpus is turned back, and, consequently, the wrist--that is to say, the carpus--is removed from the ground. These are the digitigrades.

Lastly, the ruminants (sheep, oxen, deer, etc.), and also the pig, a.s.s, 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.

[Ill.u.s.tration: FIG. 23.--THE HUMAN HAND RESTING ON ITS PHALANGES: LEFT SIDE, EXTERNAL SURFACE.]

[Ill.u.s.tration: FIG. 24.--THE HUMAN HAND RESTING ON THE TIPS OF SOME OF ITS THIRD PHALANGES: LEFT SIDE, EXTERNAL VIEW.]

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 pa.s.sing 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 p.r.o.nation. 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 const.i.tutes 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 h.o.r.n.y 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 p.r.o.nation--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.

[Ill.u.s.tration: FIG. 25.--SUPERIOR EXTREMITY OF THE BONES OF THE HUMAN FOREARM: LEFT SIDE, SUPERIOR SURFACE.

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

[Ill.u.s.tration: FIG. 26.--SUPERIOR EXTREMITY OF THE BONES OF THE FOREARM OF THE DOG: LEFT LIMB, SUPERIOR SURFACE.