The Skeletal System. 57 



incus is attached to the skull, while the malleus is extended as the 

 cartilage of Meckel almost the whole length of the inner surface 

 of the mandible. The bulk of the skull is formed already by 

 the surface elements distributed in the characteristic fashion, but 

 as yet only loosely associated (cf. Fig. 29). 



This condition of the developing skull in a mammal finds its 

 explanation far back in the history of the vertebrates and is only 

 made clear by the study of some one of the lower fishes such as 

 HEAD SKELETON OF ^^^""^ "J" sturgeon. In a shark (Fig. 30) 



LOWER VERTEBRATES. ^^^ entire internal skeleton is formed in 



cartilage which persists throughout life. 

 The principal part of the head skeleton is a massive cartilaginous 

 box (chondrocranium), enclosing the brain, and including as a 

 result of growth and fusion the nasal and auditory capsules. The 

 eye capsules are free, and their accommodation at 

 the side of the cranium establishes the orbital 

 depression to be seen on the skull of all vertebrates. 

 This type of structure is obviously the basis of the 

 mammalian skull, except that in the latter the 

 cartilage mass is at once more restricted to its basal 

 portion, and is more specialized by its replacement d^'tlcils' ^of^^'^th" 

 by definite bone centres. _ l^u^tdZ- e&^: 



It is, however, in respect of the visceral arches 

 suspended from the chondrocranium that the structure of the shark 

 skull is most illuminating. The first or mandibular arch forms 

 the upper and lower jaw. Its composition reflects that of the 

 external part of the auditory chain in the mammalian embryo, and 

 establishes a fact, for a long time scarcely believed by zoologists, 

 that in the history of mammals this arch has undergone a profound 

 change of function. The second or hyoid arch, though developed 

 to an extent out of keeping with the degree of development of the 

 primitive tongue, is nevertheless obviously homologous with the 

 hyoid arch of a mammal and in most sharks has an important 

 accessory function in the support of the lower jaw. Following 

 the hyoid arch are five ordinary or branchial arches supporting 

 the filaments of the gills and serving as pillars of the gill apertures. 

 These arches are then the parts of an extensive system out of which 

 those portions of the laryngeal cartilage which are of visceral rather 

 than tracheal origin have been specialized in mammals. 



