Artistic Anatomy of Animals Part 13

1, Occipital bone; 2, condyle of the occipital; 3, jugular process; 4, parietal bone; 5, parietal crests; 6, frontal bone; 7, orbital process; 8, orbital cavity; 9, external auditory ca.n.a.l; 10, zygomatic process; 11, superior maxillary bone; 12, intermaxillary or incisor bone; 13, nasal bone; 14, anterior orifice 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, incisor teeth; 20, canine teeth; 21, molar teeth.]

In the bear, the anterior margin of the frontal bone is prolonged by two small tongues of bone, which, descending on the lateral borders of the nasal bones, articulate with the superior half of the latter.

The temporal bone is, as in man, furnished with a squamous portion, from which springs the zygomatic process, which is directed towards the face, to terminate in the following manner: in the carnivora, the pig, and ruminants, it articulates with the malar bone by its inferior border; in the horse, it insinuates itself as a sort of wedge between the malar bone and the orbital process of the frontal bone, with which it articulates, as we have already pointed out, and contributes, by a portion situated in front of this articulation, to form the boundary of the anterior opening of the corresponding orbital cavity. As in man, the zygomatic process arises by two roots: one, transverse, behind which is situated the glenoid cavity of the temporal bone; the other, antero-posterior, which proceeds to join above with the superior curved line of the occipital bone.

Behind the glenoid cavity is found the external auditory ca.n.a.l, and, further back still, the mastoid process. This latter, but slightly developed in the carnivora, a little more so in the ruminants, and still more in the horse, has its external surface traversed by a crest, _the mastoid crest_, which, after becoming blended with the antero-posterior root of the zygomatic process, proceeds with this latter to join the superior occipital curved line.

Below the auditory ca.n.a.l is situated a round prominence, highly developed in carnivora; this is _the tympanic bulla_, also called _the mastoid protuberance_; it is an appendage of the tympanum.

The Face

The bone of this region, around which all the others come to be grouped, is, as in man, the superior maxillary. The relations of this maxillary with the neighbouring bones is not exactly the same in all animals; for example, in the ox, sheep, and horse, in which the bones of the nose are wide in their upper part, and in which the lachrymal bone, which is very highly developed, encroaches on the face, the superior maxillary does not meet the frontal bone; it is separated from it by the above-named bones. It unites with it, on the other hand, in the dog and the cat. In the bear, it is separated from the bones of the nose by a small tongue of bone which springs from the anterior border of the frontal--a process which we have noticed in connection with this latter.

[Ill.u.s.tration: FIG. 62.--THE SKULL OF THE OX: LEFT LATERAL ASPECT.

1, Occipital condyle; 2, jugular process; 3, parietal bone; 4, frontal bone; 5, osseous process, which serves to support the horn (horn-core); 6, orbital cavity; 7, external auditory ca.n.a.l, in front of which is found the zygomatic process; 8, temporal fossa; 9, superior maxillary bone; 10, intermaxillary or incisor bone; 11, nasal bone; 12, anterior orifice of the cavity of the nasal fossae; 13, malar bone; 14, lachrymal bone; 15, inferior maxillary bone; 16, condyle of the inferior maxillary bone; 17, incisor teeth; 18, molar teeth.]

In the pig, ox, sheep, and horse, the external surface is traversed, to a greater or less extent, by a crest which is situated on the prolongation of the inferior border of the malar bone. This crest, which is straight in the horse, but curved with its convexity upwards in the ox and the sheep, is known as _the maxillary spine_ or _the malar tuberosity_: it gives attachment to the ma.s.seter muscle, and, in the horse, is distinctly visible under the skin. It does not exist in the carnivora. On the same surface is situated the sub-orbital foramen.

The inferior border is hollowed out into alveoli, in which are implanted the superior molar and canine teeth. This border is prolonged forwards from the alveolus, which corresponds to the first molar tooth, to terminate, after a course more or less prolonged, at the alveolus of the canine. This s.p.a.ce, more or less considerably expanded, which thus separates these teeth is called the _interdental s.p.a.ce_; but this denomination is not applicable to ruminants, because these latter possess neither canine nor incisor teeth in the upper jaw (see p. 125, dent.i.tion of the ox and sheep). The superior maxillary bone of one side and that of the opposite side do not meet in the median line in the region which corresponds to the incisor teeth; they are separated by a bone which, in the human species, is present only at the commencement of life, and afterwards coalesces with the maxilla; this is the intermaxillary or incisor bone. This bone, which is paired, is formed of a central part, which bears the superior incisor teeth; it is prolonged upwards and backwards by two processes: one, external, which insinuates itself between the superior maxillary and the nasal bone, except in the sheep, in which it remains widely separated from the latter; the other, internal, which is united to that which belongs to the bone of the opposite side to form part of the floor of the cavity of the nasal fossae; the external border of this process, which is separated from the body of the bone by a notch, forms the internal boundary of the corresponding _incisor opening_ or the _incisor slit_. Owing to the absence of superior incisors in ruminants, the intermaxillary bone presents no alveoli.

The malar bone, and the os unguis or lachrymal, are more or less developed according to the species considered. With regard to the malar bone, it is most important to notice the part which it takes in the formation of the zygomatic arch, and that its inferior border contributes to form the crest to which is attached the ma.s.seter muscle.

As for the nasal bones, they present differential characters which, as they affect the form of the region which they occupy, are worthy of notice.

[Ill.u.s.tration: FIG. 63.--SKULL OF THE HORSE: 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 cavity; 8, zygomatic process of the temporal bone; 9, external auditory ca.n.a.l; 10, mastoid process; 11, superior maxillary bone or _maxilla_; 12, intermaxillary or incisor bone; 13, nasal bone; 14, anterior orifice of the cavity of the nasal fossae; 14', malar bone; 15, lachrymal bone; 16, inferior maxillary bone or _mandible_; 17, inferior maxillary fissure; 18, condyle of the inferior maxillary bone; 19, coronoid process of the inferior maxillary bone; 20, incisor teeth; 21, canine teeth; 22, molar teeth.]

Their dimensions in length are proportional to those of the face. Very small in man, they are more developed in carnivora. We recognise in the latter the two curves which characterize them in the human species, and which we clearly notice when we view them on one of their lateral aspects: a concavity above, and a convexity below. These curves are more or less accentuated--very strongly marked in the bulldog, and scarcely at all in the greyhound. Moreover, in the carnivora also the nasal bones are wider below than above, and form, by their junction, a semicircular notch which limits, in its superior portion, the anterior opening of the cavity of the nasal fossae. In the horse they present an opposite arrangement with regard to their dimensions in width; broad above, each terminates below by forming a pointed process which, separated from the intermaxillary bones, is prolonged in front of the nasal orifice.

The inferior maxillary bone is, as in man, formed of a body and two branches. But among the many special characteristics of form and size which sharply differentiate it from the human bone, one detail must be indicated; this is the absence of a mental prominence. Hence it results that the anterior border of the body of the lower jaw, instead of being directed obliquely downwards and forwards, is, on the contrary, oblique downwards and backwards, and that in certain animals this border is actually found almost exactly on the prolongation of the inferior border of the body of the bone.

On the external surface of the body are found the three mental foramina.

The superior border is hollowed out by alveoli.

With regard to the branches (_rami_), they terminate in two processes: one, the posterior, is the condyle; the other, situated more forwards, is the coronoid process, which gives insertion to the temporal muscle.

These two processes are separated by the sigmoid notch.

For reasons which we will explain further on (see p. 127, movements of the lower jaw), the condyle presents differences of form. In the carnivora, it is strongly convex from before backwards, expanded transversely, and firmly mortised in the glenoid cavity of the temporal bone; in the ruminants, it is less convex from before backwards, it is more slightly concave in the transverse direction; in the rodents--we give as an example the hare (Fig. 64)--the condyle is still convex from before backwards, but it is flattened from without inwards.

In the animals in which the muscles of mastication are very highly developed, and especially in the carnivora, the osseous regions occupied by these muscles are more extensive and more deep than in the human species. The length of the coronoid process, the depth of the temporal fossa, the extent of the zygomatic arch, the appearance of the external surface of each of the rami of the lower jaw, deeply hollowed out for accommodation of the ma.s.seter, and to provide extensive surfaces of insertion for this muscle, are sure proofs furnished by the skeleton of the occasionally enormous development of the muscles of mastication.

In the carnivora, a rather strong process, which is directed backwards, occupies the angle of the inferior maxilla; it is, accordingly, situated below the region of the condyle.

[Ill.u.s.tration: FIG. 64.--SKULL OF THE HARE: LEFT LATERAL ASPECT.

1, External occipital protuberance; 2, occipital condyle; 3, parietal bone; 4, frontal bone; 5, orbital process; 6, orbital cavity; 7, zygomatic process; 8, external auditory ca.n.a.l; 9, superior maxillary bone; 10, intermaxillary or incisor bone; 11, nasal bone; 12, anterior opening of the nasal fossa; 13, malar bone; 14, inferior maxillary bone; 15, condyle of the inferior maxillary bone; 16, incisor teeth; 17, molar teeth.]

The teeth which the jaws carry vary in number, and even in appearance, according to species; it is useful to note their differences. In order to establish the nature of these latter more effectively, we will first recall the fact that in man the teeth, thirty-two in number, are equally distributed between the jaws, and are divided into incisors, canines, and molars, of which the arrangement is thus formulated:

5_m._ 1_c._ 2_i._ | 2_i._ 1_c._ 5_m._ --------------------+-------------------- = 32.[19]

5_m._ 1_c._ 2_i._ | 2_i._ 1_c._ 5_m._

[19] _I.e._, _i_, incisors; _c_, canines; _m_, molars.

We also note that the incisors are edged, the canines are pointed, and that the molars, cubical in shape, have their surface of contact provided with tubercles.

The teeth of the cat are thirty in number; they are thus arranged:

4_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 4_m._ --------------------+-------------------- = 30.

3_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 3_m._

Those of the dog number forty-two:

6_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 6_m._ --------------------+-------------------- = 42.

7_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 7_m._

In these animals, the incisors, such as are not damaged by use, are furnished, on the free border of their crown, with three tubercles, of which one, the median, is more developed than those which are situated laterally. We denote these teeth, commencing with those nearest the median line, by the names _central incisors_ or _nippers_, _intermediate_ and _corner incisors_. The canines, or _fangs_, are long and conical; they are curved backwards and outwards. The upper canines, which are larger than those of the lower jaw, are separated from the most external of the incisors (_corner_) by an interval in which the canines of the lower jaw are received. The lower canines, on the other hand, are in contact with the neighbouring incisors, and are each separated from the first molar which succeeds them by a wider interval than that which is situated between the corresponding teeth in the upper jaw.

The molars differ essentially from the teeth of the same cla.s.s in the human species. Their crown terminates in a cutting border bristling with sharp-pointed projections; this formation indicates that these teeth are princ.i.p.ally designed for tearing. During the movement of raising the lower jaw, which is so energetic in the carnivora, they act, indeed, in the same manner as the two blades of a pair of scissors. The largest molars are: in the dog, the fourth of the upper jaw, and the fifth in the opposite one; in the cat, the third both above and below.

The pig has forty-four teeth disposed in the following manner:

7_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 7_m._ --------------------+-------------------- = 44.

7_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 7_m._

Of the incisors, the nippers and the intermediate ones of the upper jaw have their a.n.a.logues in those of the horse; in the lower jaw, the corresponding teeth, straight, and directed forward, rather resemble the same incisors in rodents. The corner incisor teeth are much smaller, and are separated from the neighbouring teeth. The canine teeth, also called _tusks_ or _tushes_, are greatly developed, especially in the male. The molars increase in size from the first to the last; they are not cutting, as in the carnivora, but they are not flattened and provided with tubercles on their surfaces of contact as in the herbivora.

In the ox and the sheep the teeth are thirty-two in number:

6_m._ 0_c._ 0_i._ | 0_i._ 0_c._ 6_m._ --------------------+-------------------- = 32.

6_m._ 0_c._ 4_i._ | 4_i._ 0_c._ 6_m._

As we see from this dental formula, the incisors are found only in the lower jaw; they are replaced in the upper jaw by a thick cartilaginous pad on which the inferior incisors find a surface of resistance.

These have their crowns flattened from above downwards, and gradually become thinner from the root to the anterior border, which is edged and slightly convex. These teeth gradually wear away. In proportion to the progress of this wear, on account of the fact that it involves the anterior borders and upper surfaces of the incisor teeth, and that these teeth are narrower towards the root than at the opposite extremity, the intervals which separate them tend to become wider and wider; and when the roots become exposed by the retraction of the gums, they are then separated from one another by a considerable interval. The molars have their grinding surface comparable to that of the horse; they increase in size from the first to the sixth.

The teeth of the horse are forty in number; they are thus distributed:

6_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 6_m._ --------------------+-------------------- = 40.

6_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 6_m._

As they become worn, these teeth continue to grow, and as, on the one hand, this phenomenon takes place throughout the whole life of the animal, and, on the other hand, the process of wear brings out and makes visible at the surface of friction parts formerly deeper and deeper, and of which the configuration varies at different levels, there result special features which permit the determination of the age of the animal by an examination of its jaws. The incisors are called, commencing with those situated nearest the middle line, _central incisors_ or _nippers_, _intermediate_ and _corner incisors_. The canines, also designated as the _fangs_, exist only in the male. It is exceptional to find them in the mare, and when they exist in this latter they are less developed than those of the horse. The molars have cuboid crowns; the surface of friction is almost square in the case of the upper molars, and is inclined so as to look inwards; in the case of the inferior ones, it is a little narrowed, and is inclined so as to look outwards. In the upper jaw the external surface of the crown is hollowed by two longitudinal furrows; in the lower jaw the same surface has only one furrow, which at times is but slightly marked.

In the hare the teeth are twenty-eight in number: