Proceeding from the consideration of the bones of the limbs we will next give particular attention to the head, mainly on account of the teeth. These, although in the popular view they are looked upon merely as organs for masticating food, and for this purpose are divided into front and back teeth or incisors and molars, do really possess very special features, by the aid of which the naturalist is enabled to determine the family or order to which animals belong.
The exploit of the palaeontologist in constructing the model of an extinct animal from a single fossil bone or tooth is often accepted as a trick of sleight-of-hand, more calculated to amuse than to instruct, but when all the facts are known there is really nothing very wonderful in the procedure. Anyone, for example, who is familiar with the form of the teeth of the shark could hardly make a mistake in their identification, and if a fossilized tooth of a shark were placed in his hand he would at once, in imagination, construct the animal to which the tooth belonged - in fact, it would be impossible for him to avoid doing so. In like manner other characteristic structures and organs are in themselves indisputable evidence of their origin, and to the naturalist the realization of the form of an animal upon such evidence is a mere involuntary and quite spontaneous mental process scarcely attended with any effort.
Fig. 660. - Side View of Skull of Man, with the bone removed so as to show the whole of the teeth.
z, Zygomatic arch ; n, nasal bone; o, orbit; t, temporal fossa; oc, occipital condyle; e, external auditory opening; g, glenoid fossa for articulation of the lower jaw; co, coronoid process of lower jaw; i1 and i2, incisor teeth; c, canine; pm1 and pm2, premolar teeth; m1 m'2 m3, the three molar teeth.
To understand the value of the evidence afforded by the teeth and certain bones of the skull of the horse as connecting the existing animal with its remote ancestors, it will be necessary to consider some of the most salient features of those structures, premising that no more than a cursory view can be taken out of respect for the patience of the reader.
If we compare the skull of man with that of the horse it will at once be evident that the difference of form is very marked, as shown in the two illustrations (figs. 660 and 661) from Sir W. Flower's book.
The letters of reference in the two figures are the same in both, and indicate the same bones. The remarkable difference in form of the two skulls is due to the variation in size and shape of the separate pieces of bone of which the cranial and facial divisions of the skull are composed.
Most noticeable is the vast difference in size of the cranium of man as compared with that of the horse. There is no difficulty in recognizing the fact that the facial division of the horse's skull, the part which is mainly used for the mastication of the food, is developed enormously out of proportion to the cranial division in which the brain is lodged - the centre of whatever degree of intelligence the animal may possess, and the source of some of the most important nerves. In man the conditions are exactly the opposite. The cranium is of immense capacity compared with the insignificant proportions of the facial bones, yet it is a fact that there are the same number of bones and a similar arrangement of them, and in short a general uniformity of the plan of construction in both cases, varied in details under the influence, it may be presumed, of the conditions of existence.
Fig. 661. - Side View of the Skull of the Horse, with the bone removed so as to show the whole of the teeth and nasal bone.
n, Nasal bone; o, orbit; z, zygomatic arch; t, temporal fossa; or, occipital condyle; e m, external auditory opening of glenoid fossa for articulation of the lower jaw; i1 i2 i3, three incisor teeth ; c, canine; pm', the situation of the first rudimentary premolar, which has been lost in the lower, but which is present in the upper jaw; pm2 pm3 pm4, the three fully-developed premolar teeth; ml m2 m3, the three true molar teeth.
With regard to the teeth of the horse some points of considerable interest have to be noticed. It has already been stated that the horse has six front teeth or incisors, named more correctly, from their flat surfaces, Hippos, in each jaw, four canine teeth (tusks) in the male, two on each side, top and bottom, while in the female the tusks are absent or are in a rudimentary condition. There are also six molars on each side, top and bottom. The last three of these are specially distinguished as the molars, those in front being known as premolars. To these must he added the first premolars, eye-teeth or wolves'-teeth, in the upper jaw, which are in a rudimentary state, and have disappeared entirely from the lower jaw. In the ancient ungulate mammals the first premolars were fairly well developed teeth, making a row of seven instead of six molars. After a gradual diminution in size, which may be traced in the fossilized remains of the ungulates of the tertiary formation, these teeth are represented in the horse of the present time only by the small conical teeth in front of each of the first well - developed molars of the upper jaw. These teeth (the eye-teeth) - which, as is well known, were once (and are still by some) looked upon as a cause of blindness, and were always punched out as soon as discovered - are undoubtedly therefore vestigial remains, and in course of time may cease to appear altogether in the horse's mouth. They are usually got rid of between two and three years of age, when the two first molars are exchanged for the second teeth or permanent molars.
Between the corner incisors and the first molars is a clear unoccupied space, the diastema, popularly called the bar. This toothless space did not exist in the most ancient mammals, but in the primitive equine ungulates there were some indications of it, and the feature becomes more and more distinct through the whole series of horse-like animals. What circumstances led to the change, or what object is gained by it, is not known, but in the horse the space in the lower jaw is taken advantage of for the purpose of adjusting the bit. The incisors and molars of the horse are remarkable for their complicated structure. Three materials of different degrees of density may be distinguished in their formation. Of these the least dense is known as the crusta petrosa or cementum; the next in hardness forms the bulk of the tooth, and is called dentine; the hardest of the three is the enamel which covers the dentine, following it in all its convolutions. The crusta petrosa covers the root of the tooth; it consists of lamellated bone tissue, with lacunae and canaliculi, but without haversian canals. It is covered with periosteum, which is also reflected on to the walls of the alveolar cavity.
In the incisor tooth of the horse there is no external dentine; the crown of the tooth is enamel and crusta petrosa (fig. 662). In the molar tooth affairs are different; crusta petrosa, dentine, and enamel all come to the surface, and it is owing to the different degrees of hardness of these substances that the necessary roughness of surface is maintained, as the crown is worn away by use.
Not only do the three structures of the teeth accomplish an important purpose in the preparation of the food for digestion, but owing to variation of colour they present a characteristic appearance which, taken in connection with the form of the teeth, constitutes a distinguishing feature in the Equidoe or horse family easily recognized and quite reliable.
With the above sketch of some of the special parts of the skeleton of the horse it will be convenient to proceed to the examination of some other characteristics which are exhibited by the animal.
Colour may attract the attention of an observer even more than variations in size and outline, and in many instances the peculiarities which are recognized on the surface are more definite, and certainly to the ordinary observer more intelligible, than the evidence which the anatomist obtains from a close examination of internal parts.
For example, some six or seven species of the equine family may be distinguished by mere inspection of the markings on certain parts of the skin, the arrangement of the hair of the mane and the tail, the size and the shape of the feet, the length of the ears, and the existence or the absence of small horny callosities on the fore and hind extremities.
The horse (Equus caballus, as it is known to naturalists) is variously coloured, and to the observant eye has very curious markings, often spots or stripes here and there. On the tail the hair is long, and grows from the root to the tip of the organ, covering it completely. The mane also is eminently characteristic, especially that part of it, the forelock, which hangs between the ears over the forehead.
Peculiar markings are also frequently seen in the form of a spot, light or dark in colour, on the centre of the forehead. This mark varies in form, is circular or oval, elongated or lozenge-shaped, and sometimes occupies a considerable space, forming a kind of shield over the whole front of the face. Blaze or race is the term given to this mark.
Fig. 662. - Section of Unworn Incisor of Adult Horse.
a, Dentine; b, Enamel; c, Ce-mentum;d, Pulp cavity.