28 COMPARATIVE ANATOMY. 



called membrane bones; and may form either in the peri- 

 chondrium, or, as in the case of certain cranial bones, in 

 the membranous roof continuous with the perichondrium. 

 (Huxley.) In conjunction with the osseous skeleton are 

 often found portions of a cartilaginous framework more or 

 less complete. This, for the most part, is confined to the 

 cranium, and is best expressed in the heads of Batrachia and 

 certain fishes, as in Rana (frog), and Esox (pike). With the 

 higher Vertebrates it becomes less defined, until in man it is 

 thought to be represented in the small cartilage occupying 

 the median lacerated foramen at base of skull. This carti- 

 lage, together with a few others, such as the tracheal and 

 auricular cartilages, never ossifies. 



The study of the osseous vertebrate skeleton, with a view 

 of instituting comparisons between it and that of man, should 

 at all times be pursued in conjunction with the study of its 

 development. In no other way can the construction of the 

 skeleton be understood. As an approximation to truth it 

 may be stated, that every centre of ossification stands for a 

 bone, and therefore that there are as many bones in a skel- 

 eton as centres of ossification. The actual number in the 

 adult skeleton of the majority of animals, however, is much 

 smaller than its centres of development: a result obtained 

 from the tendency of centres to group themselves into cer- 

 tain forms which are fixed and definite within recognized 

 limits. Thus while any portion of a skeleton, resulting from 

 the union of two or more centres, is for convenience called a 

 bone, it is in reality an assemblage of bones In the osseous 

 fish there is no occipital bone; but its place is taken by a 

 number of bones called basi-occipital, ex-occipital, and supra- 

 occipital. Now in man the occipital bone is proved by de- 

 velopment to be the union of the three portions seen in the 

 skeleton of the fish, and is, therefore, not a bone, but an 

 association of bones. 



In like manner the sphenoid bone of man is divisible into 

 basi-sphenoid, pre-sphenoid, ali-sphenoid, orbito-sphenoid, 

 and internal pterygoid. The frontal, into right and left 

 frontal; to which are added, anterior and posterior frontal 



