232 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



is mainly made up of cartilage, cartilage-bones,* and membrane- 

 bones, f 



There are also a few sesamoid bones, e.g., patella, developed in the course 

 of tendons. 



The bones are distinguished by their spongy character, and 

 the shafts of the long bones contain, in the adult, air instead of 

 marrow. The skeletons of the trunk and limbs may be distin- 

 guished as axial and appendicular. 



(1) The axial endoskeleton consists of skull, vertebral column, 

 ribs, and sternum. (a) The mature skull contains compara- 

 tively little cartilage, and a chondro-cranium is only to be made 

 out in the embryo. The bones in the adult are fused together 

 so as to make the determination of boundaries a difficult matter. 

 A rounded cranial portion behind may be distinguished from a 

 tapering facial portion in front, the two being connected together 

 by an imperfect joint which permits a small amount of up and 

 down movement. A large rounded orbit is present on each side 

 near the junction of the two. 



The cranial part is mostly made up of the rounded brain-case 

 or cranium. This exhibits a large foramen magnum behind, 

 bounded by the occipital region, to form which four bones are 

 fused together, two ex-occipitals at the sides, a supra-occipital 

 above, and a basi-occipital below. The last bears a median 

 rounded occipital condyle, for articulation with the vertebral 

 column. The roof of the brain-case is completed by two pairs 

 of bones, the parietals behind and the frontals anteriorly. The 

 side-wall of the brain-case is partly formed by the squamosal, 

 below which is a depression, the tympanic cavity, bounded 

 internally by the auditory capsule. 



This is formed by the union of three elements, pro-otic, epi-otic, and 

 opisth-otic, of which the first is most important. They are placed in front, 

 above, and behind respectively. 



About the centre of the capsular wall are two small openings, 

 one above the other, separated by a very narrow interspace. 



* Cartilage-bones : Basi-, ex-, and supra-occipitals, pro-, epi-, and 

 opisth-otics, basi-, pre-, alt-, and orbito- sphenoid*, meftethmoid, quadrate, 

 articular, columella, basi-hyal, basi-branchiaf, posterior cornua of hyoid. 

 Vertebrae. Sternum. Ribs. Appendicular skeleton (except furcula). 



f Membrane-bones : Parietal, frontal, squamosal, parasphenoid, lachry- 

 mal, basi-temporal, vomer, nasal, premaxilla, maxilla, jugal, quadrate- jugal, 

 pterygoid, palatine, angular, supra-angular, dentary, splenial. Furcula. 



