OSSEOUS SYSTEM. O 



OSSEOUS SYSTEM. 



THE type in the Skeleton of the mammiferous animal is that of 

 the Vertebrata generally, and of Man in particular. It nevertheless 

 presents remarkable varieties of form in the class, which, however, 

 are all easily understood, when they are regarded either as modifica- 

 tions of the human skeleton, or are viewed in relation with the ele- 

 ment in which the animals live. The Cetacea and Edentata are 

 farthest removed from the human type, then the Cheiroptera, Rumi- 

 nantia, Pachydermata, Marsupiata, and some Insectivora, still less 

 the Rodentia and Carnivora, and least of all the Quadrumana. Ac- 

 cording as the animals live in water and move by swimming, or in- 

 habit cavities dug in the earth, or are organized for running or flying, 

 or can use the extremities for seizing and tearing, will the skeleton 

 be modified throughout, and the extremities, along with the bony 

 arches which support them, be lengthened, shortened, or otherwise 

 altered, until at length a very evident relationship is established with 

 Fishes, Amphibia, and Birds. 



The Cranium of the Mammalia, as regards the number and ar- 

 rangement of its individual bones, agrees in all points with that 

 of Man, and possesses certain peculiarities which distinguish it from 

 the two next classes of Birds and Amphibia. The lower jaw al- 

 ways articulates by a single more or less convex condyle, with the 

 skull ; and the intermediate bone, called os quadratum, which is 

 present in the other Vertebrata, is here absent. The facial bones 

 are immovably connected to each other, while those of the cranium 

 form a rounded skull, which is developed in an inverse ratio to the 

 former. The sutures of the bones of the skull generally remain visi- 

 ble throughout life, though with age, and in particular orders, they 

 exhibit a tendency to become obliterated. The occipital bone is 

 constantly, as in Man, articulated by means of two condyles with 

 the atlas, and is divided in the embryo into a basilar, two condy- 

 loid, and a posterior portion, all of which remain permanently de- 

 tached in the lower classes of Vertebrata. The foramen magnum 

 is usually situated quite at the back part of the skull, and in a posi- 

 tion more or less perpendicular. It is only among a few of the 

 Apes, and especially the young animals, that it approaches the hori- 

 zontal direction which it has in Man, by being advanced more 

 toward the sinciput. It is frequently small, and more or less tri- 

 angular or quadrangular, as in the Pachydermata, sometimes, as 



