Artistic Anatomy of Animals Part 14

6_m._ 0_c._ 2_i._ | 2_i._ 0_c._ 6_m._ --------------------+-------------------- = 28.

5_m._ 0_c._ 1_i._ | 1_i._ 0_c._ 5_m._

The four incisors of the upper jaw are divided into two groups; one of these is formed by the two princ.i.p.al teeth, the other by two very small incisors which are placed behind the preceding.

Having studied the jaws and examined the arrangement of the teeth, we should say a few words on the movements which the lower jaw is able to execute. In man, these movements are varied in character: the jaw is lowered and raised; it can also be projected forwards and drawn backwards, or carried to the right or left side by lateral movements.

Owing to the different modes of nutrition of animals, with which the shape of the teeth is clearly correlated, being more specialized than in the human species, the lower jaw is moved in a fashion less varied and in the direction most suitable for the mastication of the foods which form the aliment of the species considered. Moreover, this is plainly shown in the skeleton by the shape of the condyle of the lower jaw (see p. 122, different forms of this condyle). In the carnivora, whose teeth, as we have seen, are all cutting ones, the jaw rises and falls; the food then is, if we consider the two jaws, cut as by the blades of a pair of scissors. In the ruminants, the incisors exist only in the lower jaw, but the molars are thick and well developed; the food is ground by these latter as by millstones, and the movements which favour this action are, above all, the lateral. As for the rodents, in which the incisors are formed for filing down and cutting through hard resisting bodies, their lower jaw moves in the antero-posterior direction, in such a way that the inferior incisors alternately advance and recede beneath those of the upper jaw. The free cutting border of these teeth effectively fulfils the function to which they are destined; their constant wear preserves and revivifies the chisel edge which characterizes them, without leading to their destruction, for the incisors in rodents are of continuous growth.

THE SKULL OF BIRDS

=The Skull of Birds= (Fig. 65).--If, because it is less important from the artistic point of view, we do not consider it necessary to describe in detail the skull of birds, we yet think it useful to indicate, in their general lines, the peculiarities it presents.

In this group the skull is generally pear-shaped; to the cranium, of which the bones are arranged in such a way as to give it a form more or less spherical, succeeds a face more or less elongated, according as the bill is more or less developed.

In general, the bones of the skull coalesce very early, with the result that it is only in very young individuals that we can determine their presence.

We find the skull to consist of an occipital bone, two parietals, a frontal, etc.; we will indicate but one detail in connection with these bones: it is the presence of a single condyle for the articulation of the occipital bone with the atlas. We also note the quadrate bone, which is situated on the lateral part of the cranium, is movable on this latter, and acts as an intermediary between it, the bones of the face, and the lower jaw. The quadrate bone is regarded as a detached portion of the temporal; on the signification of this we do not now propose to dwell.

[Ill.u.s.tration: FIG. 65.--SKULL OF THE c.o.c.k: LEFT LATERAL SURFACE.

1, Occipital bone; 2, parietal bone; 3, frontal bone; 4, ethmoid bone; 5, cavity of the tympanum; 6, quadrate bone; 7, superior maxillary bone; 8, malar bone; 9, nasal bone; 10, 10, intermaxillary bone; 11, nasal orifice; 12, os unguis or lachrymal bone; 13, inferior maxillary bone.]

On the anterior portion of the face we find the nasal bones, which, articulating with the frontal on one side, circ.u.mscribe, on the other, the posterior border of the nares. The nasal bone of the one side is separated from that of the opposite by the intermaxillary or premaxillary bone, which forms the skeleton of the superior mandible.

The superior maxillaries, which are rudimentary, are situated on the lateral parts, and prolonged backward by an osseous style which articulates with the quadrate bone; this styloid bone, the h.o.m.ologue of the malar, is designated by certain authors as the _jugal_ or _quadrato-jugal_ bone.

It is with the quadrate bone also that the inferior maxillary articulates.

CHAPTER II

MYOLOGY

The first point to decide in commencing this study is the order in which we shall consider the different muscles which we have to examine. It must not be forgotten that in the present work we compare the organization of animals with that of man, which we already know, and that it is on the construction of this latter that, in these studies, the thought must at each instant be carried back in order to establish this comparison. Now, the general tendency which we notice in our teaching of anatomy, when one regards the region of the trunk in the human figure (a living model or a figure in the round), is first to consider the anterior aspect. It is the latter that, for this reason, we study at the very beginning; we next deal with the posterior surface of the trunk, because it is opposite; lastly, the lateral surfaces, because they unite with the preceding surfaces, the one to the other.

In studying an animal, it is usually by one of its lateral aspects that one first observes it; it is, in fact, by these aspects that it presents its greatest dimensions, and that the morphological characters as a whole can be more readily appreciated. Hence, possibly, the order of description adopted in most texts, or in the figures which accompany them. The first representation of the human figure as a whole, in a treatise on anatomy, represents the anterior aspect; the first view of the horse as a whole, in a treatise on veterinary anatomy, for example, is, on the other hand, a lateral view.

We break with this latter custom, and, without taking into account the tendency above indicated, we will commence our a.n.a.lysis with the study of the aspect of the trunk, which corresponds to the anterior aspect of the same region in man.

The first muscles usually presented for study to artists being the pectorals, it is their h.o.m.ologues that we will first describe here. We will afterwards describe the abdominal region, then the muscles which occupy the dorsal aspect of the trunk. With regard to the lateral surfaces, they will be found, by this fact alone, almost completely studied, since the muscles of the two preceding (back and abdomen), spreading out, so to speak, over them, contribute to their formation.

Nothing further will remain but to incorporate with them the muscles of the shoulder; but these will be studied in connection with the anterior limbs, from which they cannot be separated.

The neck, in man, may be considered in an isolated fashion, because, on account of its narrowness in proportion to the width of the shoulders, it is clearly differentiated from the trunk; for this reason we combine the study of it with that of the head. In animals, because of the absence or slight development of the clavicles, the neck is generally too much confounded with the region of the shoulders to make it legitimate to separate it from that region in too marked a fashion. It will, accordingly, be considered next.

We will then undertake the study of the muscles of the limbs, and end with the myology of the head.

THE MUSCLES OF THE TRUNK

We shall divide them into muscles of the thorax, of the abdomen, and of the back.

Muscles of the Thorax

=The Pectoralis Major= (Fig. 66, 1, 2; Fig. 67, 3, 4; Fig. 68, 7; Fig.

69, 10; Fig. 70, 11).--Further designated by the name of _superficial pectoral_, this muscle is described in treatises on veterinary anatomy as formed of two portions: an anterior one, called the _sterno-humeral_ muscle; the other, situated below and behind the preceding, bearing the name of _sterno-aponeurotic_.

It occupies the region of the breast, and, as a whole, it takes origin from the median portion of the sternum, from which it is directed towards the arm and forearm.

The anterior portion (sterno-humeral muscle)--thick, forming an elevation under the skin, and really const.i.tuting the pectoral region--is directed downwards and outwards to be inserted into the anterior margin of the humerus--that is to say, to the ridge which limits in front the spiral groove of this bone.

The other part (sterno-aponeurotic muscle) is situated more posteriorly, and corresponds to the region known in veterinary anatomy as the _inter-fore-limb s.p.a.ce_, which is limited laterally on each side by the superior portion of the forearm, of which the point of junction with the trunk bears the name _ars_. Arising from the sternum, as we have above indicated, this portion is directed outwards, to be joined with the terminal aponeurosis of the sterno-humeral, and with that which covers the internal surface of the forearm.

All things considered, the sterno-humeral muscle may be regarded as the representative of the upper fibres of the great pectoral of man, of which the attachments, owing to the more or less complete absence of the clavicle in the domestic mammals, the fibres must be concentrated on the sternum; the sterno-aponeurotic portion then representing the inferior fasciculae of the same muscle.

[Ill.u.s.tration: FIG. 66.--MYOLOGY OF THE HORSE: ANTERIOR ASPECT OF THE TRUNK.

1, Pectoralis major (sterno-humeral); 2, pectoralis major (sterno-aponeurotic); 3, mastoido-humeralis; 4, point of the shoulder; 5, sterno-mastoid or sterno-maxillary: 6, inferior portion of the platysma myoides of the neck, divided; 7, triceps cubiti; 8, brachialis anticus; 9, radialis (anterior extensor of the metacarpus); 10, scapular region.]

The great pectoral muscle of one side is separated from that of the opposite side along the median line, and especially above and in front, by a groove which is more or less deep, according as the muscles are more or less developed. At the bottom of this groove, suggestive of that which exists in the corresponding region in man, is found, as in this latter, the median portion of the sternum.

The preceding description particularly applies to the arrangement which the great pectoral presents in the horse; in other animals it is marked by some distinctive characters. In the pig, it is inserted into the sternum as far only as the level of the third costal cartilage; in the ox and sheep, it extends as far as the sixth; in the dog, it is attached to the two first sternal pieces only--that is to say, as far as the third costal cartilage. Moreover, in the latter, as in the cat, the two portions which we have indicated are less readily distinguished.

The great pectoral, by its contraction, draws the fore-limb towards the middle line--that is to say, adducts it.

=The Pectoralis Minor= (Fig. 67, 6; Fig. 68, 8; Fig. 69, 11; Fig. 70, 12, 26).--This muscle, also called the _deep pectoral_, is, in animals, larger than the superficial pectoral, therefore certain authors prefer to give to this muscle and the preceding one the names of deep and superficial pectoral respectively. This nomenclature is evidently legitimate, and conforms more to reality, since it does not bring in the notion of dimensions which here is found in contradiction to nomenclature; but, in order to establish more clearly the parallelism with the corresponding muscles in man, we think it better, nevertheless, to give them the names by which it has been customary to designate them in connection with the latter.

We will recall at the outset that the lesser pectoral muscle in man is completely covered by the great. In animals this is not the case; the lesser pectoral being very highly developed, projects beyond the great pectoral posteriorly, and occupies to a greater or less extent the inferior surface of the abdomen.

It also consists of two parts: one anterior, which we designate by the name of _sterno-prescapular_; the other, posterior, bearing that of _sterno-humeral_.[20]

[20] This division of the pectorals certainly complicates the nomenclature of these muscles; nevertheless, it introduces no insuperable difficulty from the mnemonic point of view. But where the study becomes less profitable, and comparison with the corresponding muscles in man more complicated, is in adopting the nomenclature of Bourgelat. Indeed, the great pectoral is designated by this author the 'common muscle of the arm and forearm,' while the lesser pectoral (or deep pectoral) is called the 'great pectoral' in its sterno-trochinian and 'lesser pectoral' in its sterno-prescapular portion. We do not consider it necessary to give the other theories relative to the h.o.m.ologies of these, notwithstanding the very real interest which they present from the purely anatomical point of view, as they have but few applications in the study of forms.

The sterno-prescapular muscle, being covered by the sterno-humeral, has little interest for us. It arises from the sternum, and is directed towards the angle formed by the junction of the scapula and humerus; then it is reflected upwards and backwards, to terminate on the anterior margin of the shoulder by insertion into the aponeurosis, which covers the supraspinatus muscle.

We can, especially in the horse after removal of the skin, recognise it, at the level of this region, in the inters.p.a.ce limited by the superficial muscles (Fig. 70, 26).

In the dog and cat this portion of the muscle does not exist. The other division of the muscle, the sterno-trochinian, is more interesting. It arises from the abdominal aponeurosis and the posterior part of the sternum; hence it pa.s.ses forward, turns under the superficial pectoral, and is inserted into the lesser tuberosity (trochin) of the humerus.

In the pig, dog, and cat, it is inserted into the greater tuberosity (trochiter) of the bone of the arm.

The superior border of this muscle is in relation with a superficial vein, which is distinctly visible in the horse--the subcutaneous thoracic vein, which in this animal is called the vein of the spur.

The sterno-humeral muscle, in contracting, draws the shoulder and the whole anterior limb backwards.

=Serratus Magnus= (Fig. 67, 2; Fig. 69, 8; Fig. 70, 9).--This muscle, which is situated on the lateral aspect of the thorax, is covered to a considerable extent by the shoulder, the posterior muscular ma.s.s of the arm, and by the great dorsal muscle.