CLASS MAMMALIA 



635 



The axial skeleton consists, as in the pigeon, of a skull, ribs, 

 sternum, and vertebral column. The skull (Fig. 511) is formed 

 of both cartilage- and membrane -bones, and only a small amount 

 of cartilage. The individual bones are immovably united to one 

 another, and their boundaries are in many cases obliterated in 

 the adult and can only be made out in the embryo. The follow- 

 ing points are 

 worthy of special 

 mention. The 

 occipital ring is 

 completely ossi- 

 fied and there are 

 two occipital con- 

 dyles (Fig. 511, 

 20) ; the cranial 

 and olfactory 

 cavities are sepa- 

 rated by a bony 

 cribriform plate ; 

 the lower jaw (if) 

 articulates di- 

 rectly with the 

 squamosal (9) ; 

 three small but 



distinct auditory ossicles are present; and there is no distinct 

 parasphenoid on the under surface. 



The teeth are cutaneous structures, as are the scales and teeth 

 of the dogfish-shark (p. 424), and are developed from the mucous 

 membrane of the mouth. Each tooth possesses an outer, hard 

 covering, called enamel, a central softer substance, called den- 

 tine, and about the base and in the surface folds a bony layer, 

 the cement. The teeth of the rabbit remain open at the base and 

 continue to grow throughout life, thus supplying new material 

 to replace that worn away in grinding its vegetable food. 



The rabbit lacks canine teeth, and the incisors (Fig. 511, 14, 16) 



Fig. 511. — Side view of skull of the rabbit. /, nasal 

 bone; 2, lachrymal bone; 3, orbito-sphenoid; 4, frontal; 

 5, optic foramen; 6, orbital groove for trigeminal nerve; 

 7, zygomatic process of squamosal; 8, parietal; g, squa- 

 mosal; 10, supra-occipital; //, tympanic bones; 12, ex- 

 ternal auditory meatus; 14, lower incisor; 75, anterior 

 premolar; 16, anterior upper incisor; 17. mandible; 

 iS, maxilla; iq, premaxilla; 20, occipital condyle. 

 (From Shipley and MacBride.) 



