138 STRUCTURE OF THE VERTEBRATES 



column. In other words, the mouth and face have been sub- 

 merged under the calvarium. A comparison of the opossum, 

 lemur, monkey, anthropoid ape, a primitive man, and the present 

 man will demonstrate this line of evolution. 



The Human Skull. The early embryology of the human fol- 

 lows the stages outlined under the development of the chon- 

 drocranium. As bone cells begin the replacement of cartilage, fine 

 strands of bone can be seen in the deeper layers of the skin. 

 These bone strands begin at definite centers, one for each bone 

 of the skull. As the bony area spreads, the dermal bones come in 



Frontal 



Parietal 



Parietal 



Occipital 



Top View 



^^^— ■^\ ^'^^^^^^^ ^Occipital 



Squamosal 

 ympanicring 

 2ygorm 



Side View 



Fig. 75. Skull of a new born Human Infant. 



contact at their rounded margin, and naturally leave spaces at 

 the junction of four rounded corners. These spaces are the jon- 

 tanelles. The "soft spot" on top of the young human is the fon- 

 tanelle formed by the juncture of the parietals and frontals. 



In structure the human skull is only a shortened, dorsally en- 

 larged mammal skull. As in other mammals, spirals of bone have 

 grown into the nasal cavities, arising from the ethmoids and the 

 dermal bones. These serve the function of moistening and warm- 

 ing the air, and on them are located the olfactory sense endings. 

 The olfactory nerves pass back toward the brain as a number 

 of small nerves, each piercing the ethmoid bone which forms the 

 anterior of the calvarium. This sieve-like bone is the cribriform 

 plate. The nerve foramina are packed into a smaller area than in 



