THE SKULL 55 



certain cartilages of the larynx, with which the hyoid is intimately 

 associated. The relation of this complex to the skull is indicated 

 in a mammal by the suspension of the hyoid apparatus from its 

 base. The three bones of the auditory chain (the series of small 

 bones in the middle ear — malleus, incus, and stapes) and the 

 elements just referred to, together with certain replacing or derm 

 elements, constitute the modified remnant in the mammal of what 

 is often regarded as a third main division of the internal skeleton, 

 namely, the visceral skeleton. 



Fig. 31. Lateral view of skull of rabbit foetus, 45 mm: cb, co, en, 

 cranial, orbital, and nasal portions of primary chondrocranium ; fr, frontal; 

 pa, parietal; pi, palatine; pmx, premapilla; sq, squamosal; st, styloid process; 

 i, incus; ip, interparietal; m, malleus; mn, mandible; mx, maxilla; na, nasal; 

 t, tympanic; zy, zygomatic. (Born plate model, after Voit.) 



Chondrocranium and Osteocranium 



The skull consists primarily in the embryo of a cartilage trough, 

 the extent of which is roughly definable as the area occupied by the 

 occipital, anterior and posterior sphenoidal, and ethmoidal portions 

 (Fig. 31). As a cartilage skull it is designated as the chondro- 

 cranium, and after its conversion into bone as the osteocranium. 

 It is no more than an enclosure foY the brain, except that it has 

 associated with it the cartilage capsules of the nasal, visual, and 

 auditory organs, and, in the case of the first and last of these, the 

 capsules are incorporated with the skull proper. This, the primary 

 skull, is designated as the neuro-cranium or cerebral cranium, to 



