VI IXTEODUCTION 



The bones of Mammals are distinguished from those of Birds by 

 the absence of air-cellsj except in some of the cranial bones, and 

 these do not communicate with the lungs. Most of them are solid, 

 or if hollow are filled with a fatty or oily matter, termed marrow. 



The cervical vertebrae are always seven in number, without excep- 

 tion, for in the Sloths, generally considered to have nine, it has been 

 lately demonstrated by Bell that the two posterior vertebrse (called 

 cervical) have rudiments of ribs attached to them, and are there- 

 fore in reality modified dorsal vertebrse. The occipital bone arti- 

 culates with the atlas, or first vertebra, by two lateral condyles. 



The head is made up of numerous bones, which are divided into 

 the cranial, or those inclosing the brain, and the facial. The nor- 

 mal number of facial bones is seventeen, viz., 2 nasal bones, 2 upper 

 maxillary, 2 intermaxillary, 2 lachrymal bones, 2 inferior turbinated 

 bones, 2 palatal bones, 2 jugal or malar bones, 2 lower-jaw bones, 

 and 1 vomer. Of these the nasal, upper and iritermaxillary, and 

 the palatal bones bound the nasal cavity, and constitute the bony 

 palate. The cranial bones are eleven in number, viz., 2 frontal, 2 

 parietal, 2 squamous, and 2 tympano-petrous bones (which together 

 make up the two temporal bones), 1 occipital, 1 sphenoid, and 1 

 ethmoid. 



The orbit is bounded anteriorly by the molar and lachrymal bones, 

 and its posterior boundary is generally absent. The orbital cavity 

 is formed by processes from the frontal bones, the lachrymal, the 

 molar, and sphenoid bones ; the ethmoid and palatine bones occa- 

 sionally assisting. The ethmoid, the turbinated bones, and the vomer 

 are internal, connecting the nasal bones with the base of the skull. 



The lower maxillary bones, united at the chin, are moveably 

 articulated with the temporal bone by a convex condyle, in this 

 differing from Birds and Reptiles, in which the articulation takes 

 place through a separate piece, the tympanic bone. In Man and 

 many Mammals several of these bones are united, viz., the frontals, 

 the parietals in some Mammals, and the temporal bones. 



The number of dorsal vertebrse depend on that of the ribs, and 

 varies from eleven to twenty ; and the lumbar, sacral, and coccygeal 

 vertebrse also vary from four to forty-five. 



