By flexion, the inferior extremity of the femur is directed forwards; the bone of the thigh then takes a more oblique direction than the normal. This movement takes place, for example, when the animal carries forward one of its hinder limbs. Extension, which takes place in an inverse sense, is produced when the foot is fixed on the ground, while the body is projected forward. It is also produced in the action of kicking.
As for the lateral movements--viz., abduction and adduction--they are less extensive than the preceding movements. The absence of the pubio-femoral ligament in other quadrupeds than the horse explains why in them abduction is less limited than in the latter. Indeed, it is the tension of this ligament, occasioned by the abduction of the thighs, which arrests more quickly the movement in question.
=Articulation of the Knee.=--This articulation, as in man, is formed by the femur, the patella, and the tibia.
In the horse the ligament of the patella is not single, but consists of three parts, designated, on account of their position, by the respective names of external, internal, and median patellar ligaments. The two former come from the angles on the corresponding borders of the knee-cap; the median springs from the anterior surface and inferior angle of the same bone. They all three pa.s.s to their termination on the anterior tubercle of the tibia. The external ligament is the strongest, and the internal ligament the least developed.
In the dog, the cat, the pig, and the sheep, the patellar ligament consists of a single band. The articulation is further strengthened on the sides by lateral ligaments--an internal and an external.
With regard to the princ.i.p.al movements, these are flexion and extension, to which may be added movements of rotation of limited extent. In flexion, the leg bends on the thigh; its inferior extremity is directed upwards and backwards; the angle which the tibia naturally forms with the femur becomes less obtuse.
But it should be understood that one part of this description--that which has relation to the leg--holds good only when the femur is in its normal condition, or in flexion. Indeed, at the close of the movement in which, during a step, the foot is in contact with the ground--that is, at the termination of the resting stage--the inferior extremity of the tibia is directed backwards. But the femur is then in a state of extension, and in regard to this latter the att.i.tude of the leg is unchanged.
[Ill.u.s.tration: FIG. 55.--EXTENSION OF THE LEG: RIGHT POSTERIOR LIMB OF THE HORSE, EXTERNAL SURFACE. (AFTER A CHRONOGRAPHIC STUDY BY PROFESSOR MAREY.)]
At this moment, notwithstanding the direction, which recalls that which it has at the time of flexion, the leg is not bent on the thigh; on the contrary, it is almost in the line of its continuation (Fig. 55). As we have done in connection with the articulations of the anterior limbs, we borrow this figure from the interesting chronophotographic studies of Professor Marey.[17]
[17] E. J. Marey, "a.n.a.lysis of the Movements of the Horse by the Chronophotograph" (_La Nature_, June 11, 1898).
=The Tibio-tarsal Articulation and of the Bones of the Tarsus.=--In the region which veterinary anatomists call the ham, the articulations of the leg and foot alone call for special study in the case of the horse.
The articulations of the bones of the tarsus, and of these with the metatarsus, do not offer any interest with regard to mobility, this being almost wholly absent at that level.
The leg and the astragalus, in a general way, are placed in contact by such articular surfaces that the resulting joint, which is a true hinge, permits movements of flexion and extension only. Indeed, as we have indicated above, the tibia is furnished, on the inferior surface, with a crest that fits into the deep groove which is situated on the corresponding surface of the astragalus.
During flexion, the anterior surface of the foot tends to approach the anterior surface of the leg, the angle formed by these two segments becoming more and more narrowed. The displacement in the opposite direction characterizes extension.
In other quadrupeds, the articulations which bind together the bones of the tarsus possess a little more freedom of movement. The shape of these bones, and particularly the shape of the surfaces of the astragalus, which are in contact with them, allow movements in this region, in the case of the dog and cat, which, without being so extensive as those of the human foot, in the subastragaloid articulation, nevertheless, recall the mobility which we find in the human species at this level--that is to say, rotation, abduction, and adduction of the foot.
As for the articulations of the metatarsus with the phalanges, and of the phalanges with one another, they resemble those of the anterior limb too closely that it should be necessary to study them here. Such a study would be, in this case, but a repet.i.tion (see p. 76, a description of the articulations in question).
THE HEAD IN GENERAL, AND IN SOME ANIMALS IN PARTICULAR
When we compare, by the examination of one of their lateral aspects, the skull of man and the same region in other mammals, it is easy to observe that the relative development of the cranium and face is entirely different. In the case of man the cranium is large, and the face relatively small; in animals the face is proportionally much more highly developed. The measure of the facial angle permits us to note these differences, and the figures relative to the value of this angle are sufficiently demonstrative to induce us to indicate those which are, in a general way, connected with some of the forms in individuals which here occupy our attention. In the first place, we must remember that the angle in question is more acute, as the cranium is less developed in proportion to the facial region (Figs. 56 and 57). It is especially to this character that we wish to draw attention.
Man 70-80 Cat 41 Dog 28-41 Sheep 20-25 Ox 18-20 a.s.s 12-16 Horse 11-13
Besides, in animals the cranium is very prominent superiorly, and the face, more or less elongated, is sharply projected downwards and forwards; in man the cranial region occupies not only the superior, but also the posterior part; the face is short and of a compact form. The human head, in its general aspect, may be compared to a sphere, while the skull of the quadrupeds presents the aspect of a quadrangular pyramid, with the base turned upwards and the summit at the incisor teeth.
=Direction of the Head.=--Before entering on the study of the bones of the head, it is necessary, in our opinion, to agree as to the position in which we shall suppose it to be placed.
The question may seem to be one of little importance; nevertheless, it cannot be regarded as indifferent, since authors are not all agreed on this subject.
Some suppose it to be placed vertically--that is, with the incisor teeth turned directly downwards. Others, on the contrary, suppose it to be placed horizontally, resting on the whole length of the lower jaw, the face being then turned upwards. These two extreme methods of arrangement appear to us to possess inconveniences--at least, for comparison with the human head.
[Ill.u.s.tration: FIG. 56.--HUMAN SKULL: MEASURE OF THE FACIAL ANGLE BY THE METHOD OF CAMPER. ANGLE BAC = 80.
The internal wall of the cranial cavity is marked by the dotted line.]
[Ill.u.s.tration: FIG. 57.--SKULL OF THE HORSE: MEASURE OF THE FACIAL ANGLE BY THE METHOD OF CAMPER.[18] ANGLE BAC = 13.
The internal wall of the cranial cavity is shown by the dotted line.]
[18] We have indicated on this sketch of the skull of the horse the facial angle measured by the method of Camper, in order that the correspondence with Fig. 56 may be more complete. But it is certain that the procedure here employed is in practice not satisfactory, since the apex of the angle, as we can demonstrate, is found to be situated within the contour of the head, and that, consequently, it is rather difficult to localize it precisely in the case of a given skeleton. Further, because of the absence of the base of the nose in the complete skull, the auriculo-nasal line cannot be accurately fixed. It would be the same for most other animals. This is why the method employed for these latter is preferably that of Cuvier, or, again, that of Cloquet. In the former, the apex of the angle of Camper is transferred to the free border of the upper incisors, but these teeth may be absent, and, on the other hand, ruminants are dest.i.tute of them. In the second, the same apex is placed at the alveolar border, and the angle then becomes fairly easy to appreciate.
Indeed, if, when the head is vertical, the same regions of the face (forehead, nose) are, in the case of animals as well as man, turned forward, the lower jaw ceases to merit its appellation, as it is then situated, not below, but behind the upper. Furthermore, if this position is chosen, for example, for modelling or drawing, it cannot be obtained without difficulty when we have to deal with an isolated piece of the skeleton, on account of the absence of equilibrium, which it is necessary to obviate. It is true that the question of convenience should not take precedence of all others, and it suffices for us in this connection to recall, in regard to the human pelvis, that, although the older anatomists used to represent it as resting commodiously on the three angles which terminate it at its lower part (ischial tuberosities and coccyx), this att.i.tude being false, it is customary now to incline the superior aspect forwards, inasmuch as this arrangement more nearly conforms to reality, in spite of the fact that it is a little more difficult so to dispose an isolated pelvis. Further, to return to the head; if its vertical direction can be demonstrated, for example, in many horses, it is not sufficiently general to be adopted as the cla.s.sic position.
In regard to the facility of placing in position, the horizontal direction is certainly to be preferred; but this is also far removed from the natural position in the animal while in the state of repose. On the other hand, the mind is not satisfied with the idea that certain regions of the face, such as the nose and the forehead, are then directed upwards. And yet it is necessary to come to a decision, seeing that what we are now investigating applies also to the position to which it is necessary to give the preference in placing the skeleton of the head when we wish to draw it in profile. That which we adopt is a compromise, but to us it seems more rational.
The position of the head of the horse, to be normal, should be such as to give it an inclination of 45. In this case the lower jaw is still posterior; and, for this reason, we see in adopting this position some inconveniences from a didactic point of view. Accordingly, we will suppose the head brought a little nearer to the horizontal, and this, from the imaginative point of view, has certainly an advantage which we cannot afford to neglect when addressing artists.
Indeed, let us suppose that to a clay model of a human head we wish to give the aspect of the head of a quadruped. We should elevate the occiput; and then, taking hold of the lower part of the face, we should lengthen it, not in a direction precisely antero-posterior, but downwards and forwards. It is obviously this latter procedure which, on the other hand, is carried out when a person wishes to give to his own face some resemblance to the muzzle of a quadruped.
It is true that, in the position we have adopted, the face is directed obliquely downwards and forwards, and that there may result a certain confusion in describing the position of its different parts. On this account, with the object of not making complications, we purpose, for the present, to subst.i.tute, for example, for the term "antero-superior"--which when speaking of the position of the forehead and nose would be more exact--the term "anterior," which is sufficiently comprehensible. The mouth will be, for the same reason, referred to as being situated at the inferior part of the face, and not the antero-inferior.
=The Skull.=--The elevation of the cranial region becomes especially appreciable when we examine the occipital bone. Before verifying this fact, it is not superfluous to recall the general arrangement which this bone presents in the human skull. A portion of the occipital bone occupies the base of the skull; but this base in man is horizontal; to this region succeeds the sh.e.l.l-shaped portion of the occipital bone, which, pa.s.sing vertically upwards, forms with the preceding portion an angle situated at the level of the external occipital protuberance, and of the curved line which starts from it on each side. In animals a portion of the occipital bone is horizontal, it is true; but this bone being sharply bent at the level of the occipital foramen and condyles, the result is that the portion which surmounts these latter looks backwards, and is limited above by the external occipital protuberance, which forms the culminating point of the skull; this point is situated between the ears.
[Ill.u.s.tration: FIG. 58.--SKULL OF ONE OF THE FELIDae (JAGUAR): LEFT LATERAL ASPECT.
1, Posterior surface of the occipital bone; 2, external occipital protuberance; 3, condyle of the occipital bone; 4, jugular process; 5, parietal bone; 6, frontal bone; 7, orbital process; 8, orbital cavity; 9, squamous portion of the temporal bone; 10, external auditory ca.n.a.l, in front of which is situated the zygomatic process; 11, tympanic bulla; 12, superior maxillary bone; 13, intermaxillary or incisor bone; 14, nasal bone; 15, anterior orifice of the nasal cavity; 16, malar bone; 17, ungual or lachrymal bone; 18, inferior maxillary bone; 19, condyle of the inferior maxillary bone; 20, coronoid process; 21, incisor teeth; 22, canine teeth; 23, molar teeth.]
[Ill.u.s.tration: FIG. 59.--SKULL OF THE LION: LEFT LATERAL ASPECT.
This figure is intended to show that in the lion the contour of the face between the nasal bones and the cranial region is more flattened than in other felidae, such as the tiger, jaguar, panther, and domestic cat. This difference is shown by comparison of this figure with the preceding one (Fig. 58). We are indebted to M. Tramond, the well-known naturalist, for the indication of this differential character which, from the artistic plastic point of view, is one of real interest.]
This protuberance, prolonged on each side by the superior curved line of the occipital bone, is so much the more prominent as this bone bends sharply a second time, so as to form a third portion, which, looking forwards, forms part of the anterior aspect of the skull, and proceeds to articulate with the parietals. On this third portion is found a crest which, proceeding from the occipital protuberance, is continuous in front with the parietal crests, to which we will again refer in speaking of the parietal bones.
On the inferior surface of the human occipital bone are found, at the level of, and external to, the condyles two bony elevations which bear the name of _jugular eminences_. They are long in quadrupeds, and const.i.tute what are designated by some authors the _styloid processes_, but they must not be confounded with the processes of the same name which in the case of man form part of the temporal bone. These processes are very highly developed in the pig, horse, ox, and sheep.
In the ox, the occipital bone is deprived of the protuberance, and is not bent on itself in the anterior portion, neither does it form the most salient part of the skull; this latter, which is situated at the level of the horns, belongs to the frontal bone. In the pig, also, the occipital bone is not bent upon itself in its anterior portion, but forms the summit of the head. The occipital protuberance, hollowed on its posterior surface, rises vertically, and rests upon the parietal bone, with which it forms an acute angle.
The parietals, two separate bones in the dog and the cat, but fused in the median line in the ox, sheep, and horse, are of special interest in regard to the two crests which, in the carnivora, and also in the pig and the horse, occupy their external surface, and, after diverging from one another, are continued by a crest which crosses the frontal bone and ends at the external orbital process of the latter bone.
These crests, known as the _parietal_ or _temporal crests_, recall both in position and relations the temporal curved line of the parietal bone of man. They contribute, as in the case of the latter, to the formation of the boundaries of the temporal fossa.
[Ill.u.s.tration: FIG. 60.--SKULL OF THE DOG: LEFT LATERAL ASPECT.
1, Posterior surface of the occipital bone; 2, external occipital protuberance; 3, occipital condyle; 4, jugular process; 5, parietal bone; 6, frontal bone; 7, orbital process; 8, orbital cavity; 9, external auditory ca.n.a.l, in front of which is found the zygomatic process; 10, tympanic bulla; 11, superior maxillary bone; 12, intermaxillary or incisor bone; 13, nasal bone; 14, anterior opening of the cavity of the nasal fossae; 15, malar bone; 16, lachrymal bone; 17, inferior maxillary bone; 18, condyle of the inferior maxillary bone; 19, coronoid process; 20, incisor teeth; 21, canine teeth; 22, molar teeth.]
In the carnivora, these crests are situated, throughout their whole length, in the median line, the temporal fossae being, accordingly, as extended as they possibly can be. In certain species, the development of these crests is such that they form by their union a vertical plate, which, in separating the two temporal fossae, gives them a greater depth.
In the pig, the parietal crests, a.n.a.logous in this respect to the temporal curved lines of the parietal bones of man, are separated by an interval, proportionately less extended, however, than that of the human skull. The parietal bone in the ox and the sheep does not enter into the formation of the anterior surface of the skull; it is formed by an osseous plate, narrow and elongated transversely, which, with the occipital bone, const.i.tutes the base of the region of _the nape of the neck_. It is bent upon itself at the level of its lateral portions so as to occupy the temporal fossa.
The anterior surface of the frontal bone, which is depressed in the median line in the dog, but plane in the horse, is limited by two crests, which, situated on the prolongation of the parietal crests, diverge more and more from one another in proportion as they occupy a lower position. This surface terminates externally in two processes, which are the h.o.m.ologues of the external orbital processes of the human frontal bone.
The superior border of these orbital processes, situated on the prolongation of the corresponding parietal crests, contributes to limit the temporal fossa. Each of these orbital processes terminates in the following manner: In the bear, dog, cat, and pig, in which the orbital cavities are incompletely bounded by bone, this process, slightly developed, is not in connection, by its inferior extremity, with any other part of the skeleton of the region. In the ox and the sheep, it articulates with a process of the malar bone. In the horse, it articulates with the zygomatic process of the temporal bone. The inferior margin of this process forms a part of the boundary of the anterior opening of the orbital cavity.
The supra-orbital foramen, which does not exist in carnivora, occupies in the horse the base of the orbital process. In the ox, it is situated a little nearer the middle line; and its anterior orifice opens into an osseous gutter which is directed upwards towards the base of the horn, while inferiorly it meets the inferior border of the frontal bone; in the sheep this groove is but slightly developed. In this latter, as in the ox, it is the frontal bone which forms the most elevated portion of the skull. In fact, being bent upon itself at a certain level, its external surface is formed of two planes: one, posterior, which is inclined downwards and directed backwards; the other, anterior, is also inclined downwards, but with a forward obliquity. At the union of these planes the bone forms an elbow, on either side of which are found the osseous processes on which the horns are mounted.
[Ill.u.s.tration: FIG. 61.--SKULL OF THE PIG: LEFT LATERAL ASPECT.