SKELETON OF MAMMILIA.
The form of the different mammalia, particularly the four footed ones, varies considerably ; and their skeletons must be marked by corresponding differen ces. Yet these varieties may be includ ed, at least for the greatest part, under the following peculiarities ; which serve to distinguish their skeletons from those of birds.
The skeletons of 'rhose of birds aremammalia possess : distinguished by : 1. A skull with 1. A skull which genuine sutures, at has not real sutures, least with very few at least in the adult. exceptions • as per haps the elephant, and the duck-billed animal, rhyncus.) 2. Jaws furnish- 2. A bill without ed with teeth. teeth.
Except the ant eaters, the mania, the balana (whale.) 3. An upper jaw, 3. An upper jaw, which does not which does move. move. There are some exceptions, viz. the rhinoceros bird.
4. An us inter- 4. No os inter.
maxillare. maxillare.
5. Two occipital 5. A single occi• condyles. pital condyle.
6. Seven cervical 6. More than se.
vertebra. ven cervical verte• Except the three- bra.
toed sloth, and some cetacea.
7. Moveable dor- 7. Motionless dor.
sal vertebra. sal vertebra.
8. A pelvis closed 8. A pelvis open in front. anteriorly.
Except the ant- Except the os.
eaters ; which have trich.
it open : and the cetacea, which have none.
9. True clavicles 9. Clavicles in a few genera stantly; and almost only. as universally the forklike bone.
The structure of the cranium presents a very remarkable singularity in the elephant. Its two tables are separated from each other, to a considerable extent, by numerous bony processes; between which are formed a vast number of cells, communicating with the throat by means of the eustachian tube, and fined with air, instead of the bloody or medullary sub stance which occupies the diploe of ani mals. The use of this structure in in creasing the surface for attachment of those large muscles, which belong to the lower jaw, proboscis and neck, and in augmenting the mechanical power of these muscles, by removing their attach ments to a greater distance from the cen tre of motion, has been very ingeniously explained by Camper. (fEuvres, tom. 2.)
These advantages are attained by the cellular structure which we have just described, without augmenting the weight of the head, and this precaution is parti cularly necessary in the present instance, as the head is on other accounts more heavy and massy in this than in any other animal. The air cells of birds, in gene ral, and particularly those which pervade the cranium in the ostrich, eagle, and owl, present examples of a similar for mation, attended with the same uses ; viz. those of increasing the bulk and strength of the hone, and diminishing its weight.
A comparison of the human cranium with that of animals will lead us to some interesting conclusions. Daubenton fix ed on the situation of the foramen mag-, num occipitale as a point of compari son. He draws two lines, which inter sect each other in the profile of the scull : one passes from the posterior margin of the great foramen, (which, in almost all mammalia, is also the superior one,) through the lower edge of the orbit ; the other takes the direction of the opening itself, beginning at its posterior edge, and touching the articular surface of the condyles, He determines, accord. ins to the angle formed by the junction of these two lines, the similarity or diver sity of the form of crania.
This angle is, however, hut an imper fect criterion ; for its variations are in cluded between and 90° in almost all quadrupeds, which differ very essentially in other points. And small variations occur in the individuals of one and the same genus.
The variations in the situations of the occipital foramen are important, when viewed in connection with the ordinary position of the animal's body. In man, who is designed to hold his body erect, this opening is nearly equidistant from the anterior and posterior extremities of the skull. The head therefore is sup ported in a state of equilibrium on the vertebral column. The angle, formed by the two lines mentioned by Dauben ton, is only of three degrees.