In human crania at least those of the foetus and young children, there is a small transverse slit near the foramen incisi vum, of which Fallopius gave the follow ing accurate account in the year 1561 "I find this division.to be rather a slit than a suture, since it does not separate one bone from the other, nor does it ap pear exteriorly, nor join two bones, which is the office of sutures." " Obs. Anat." • " Hence I was much surprised to find Vicq D'Azyr, in 1780, discover in this point an unexpected resemblance be tween the cranium of the human subject and of quadrupeds." Mem. de l'Acad. des Sc. 1780.
In the celebrated dispute of the six teenth century,whether Galen's osteology was derived from the skeleton of man or the ape, Ingrassias argued for the latter side of the question, from Galen's having ascribed an intermaxillary bone to the human subject. And the same author, in his classical " Commentarii in Galeni Li brum de Ossibus," Panorm, 1603, fol. par ticularly points out the parts, " where Ga len, led astray by the dissection of apes, deviates from the true construction of the human body." In mammalia which have horns, these parts grow oh particular processes of cer tain bones of the cranium. In the one horned rhinoceros, they adhere to a rough and slightly elevated surface of the vast nasal bone. The front horn of the two horned species has a similar attachment ; the posterior rests on the os frontis, as those of the horned pecora do. Two kinds of structure are observed in the latter ; there are either proper horns, as in the genera of the ox, goat, and ante lope ; or bony productions, as in the ge nus cervus, which includes animals of the deer kind : these are also called horns in English, or sometimes antlers ; in French, bois de cerf. In the former, the external table of the frontal bones is elongated into a process, which contains a continu ation of the frontal sinuses, except in the antelope. Its external vascular sur face secretes the horn, which covers this process like a sheath. In the stag kind (in the male only in most genera) the frontal bone forms a short flattened pro minence, from which the proper antler immediately shoots forth. It is renewed every year, and is covered, during the time of its growth, with a hairy and very vascular skin.
Castration, or any essential injury of the organs of generation,impedes the growth, alters the form, or interrupts the renewal of the horns.
The word horn, which is frequently ap plied in English to the antlers of the deer kind, as well as to the real horns of other genera, would lead to a very erroneous notion on this subject. The antler is a real bone ; it is formed in the same man ner, and consists of the same elements as other bones ; its structure is also the same.
It adheres to the frontal bone by its ba sis ; and the substance of the two parts be ing consolidated together, no distinction can be traced, when the antler is com pletely organized. But the skin of the forehead terminates at its basis, which is marked by an irregular projecting bony circle ; and there is neither skin nor peri osteum on the rest of it. The time of its remaining on the head is one year: as the period of its fall approaches, a reddish mark of separation is observed between the process of the frontal bone and the antler. This becomes more and more distinctly marked, until the connection is entirely destroyed.
The skin of the forehead extends over the process of the frontal bone when the antler has fallen : at the period of its rege neration, a tubercle arises from this pro cess, and takes the form of the future ant ler, being still covered by a prolongation of the skin. The structure of the part at this time is soft and cartilaginous ; it is immediately invested by a true perios teum, containing large and numerous ves sels, which penetrate the cartilage in every direction, and by the gradual depo sition of ossific matter convert it into a perfect bone.
The vessels pass through openings in the projecting bony circle at the base of the antler : the formation of this part, pro ceeding in the same ratio with that of the rest, these openings are_ contracted, and the vessels are thereby pressed until a complete obstruction ensues. The skin and periostenm then perish, become dry, and fall off; the surface of the antler re maining uncovered. At the stated period it falls off, to be again produced, always increasing in size.
The skeleton of quadrupeds deviates more from that of man. in the form of the lower jaw bone, than in any other part. This differ( nce consists chiefly in the want of a prominent chin ; that peculiar charaea teristic of the human countenance, which exists in every race of mankind, and is found in no other instance whatever, Man has also the shortest lower jaw in compa rison with the cranium ; the ek phant, per haps, approaching the nearest to him in this character. The same hone is further distinguished by the peculiar form and direction of its condyle. The articulation of these processes varies according to the structure of the masticating organs. They are both situated in the same straight horizontal line in the fern: ; their form is cylindrical ; and they are completely locked in an elongated glenoid cavity, whose margins are so extended before and behind the condyle, that all rotatory motions are rendered impossible, and hinge like movements only allowed. This structure is most strikingly exemplified in the badger, where the cylindrical con dyles are so closely embraced by the mar gins of the articular cavity, that the lower jaw (at least in the adult animal) is still retained in its situation, after the soft parts have been entirely removed by maceration. In many herbivorous ani mals (in the most extensive sense of the term) these condyles are really rounded eminences ; viz. in the elephant and bea ver. Their surface is flattened in the pecora, which have also the lower jaw narrower than the upper, so that the two sets of teeth do not meet together when the mouth is shut, but are brought Ili .o conta, by he free lakeral motion which takes pla,e in nimination.
As the motions of the lower jaw must be influenced by the fbrm of its condyles, uy the manner in which those processes are cinin:ctcd to the ar ticular cavity of the temporal bone, we shall find, as might have been expected, a close relation between these circum stances and the kind of food by which an animal is nourished. Thus, the lower jaw of the carnivora can only move upwards and downwards, and is completely inca pable of that horizontal motion which constitutes genuine mastication. Hence these animals cut and tear their food in a rude and coarse manner, and swallow it in large portions, which are afterwards reduced by the solvent properties of the gastric juice. Such mammalia, on the contrary, as live on vegetables, have, in addition to this motion, a power of mov ing the lower jaw backwards and for wards, and to either side, so as to pro duce a grinding effect, which is necessa ry for bruising and triturating grass, and for pulverising and comminuting grains. In all these, therefore, the form of the condyle, and of its articular allows of free motion in almost every direction. The teeth may be compared, in the for mer case, to scissars ; in the latter, to the stones of a mill.