242 THE FIFTH DAY. [CHAP. 



noid region of the skull, through which the third 

 division of the fifth nerve passes out. This cleft be- 

 comes narrowed to a small foramen. The sclerotic is 

 free, but profoundly modifies the region of the cranium 

 near which it is placed. The nasal investment is de- 

 veloped in continuity, and is closely united, with the 

 ethmoid region. 



The cartilaginous cranium, the development of 

 which has been thus briefly traced, persists in the 

 adult without even the addition of membrane bones 

 in certain fishes, e.g. the Elasmobranchii. In the Sela- 

 chioid Ganoids it is also found in the adult, but is 

 covered over by membrane bones. In all other types 

 it is invariably present in the embryo, but becomes in 

 the adult more or less replaced by osseous tissue. 



The bones in the adult skull may be divided 

 roughly into two categories according to their origin. 



(1) Cartilage bones, i.e. ossifications in the primi- 

 tive cartilaginous cranium. 



(2) Membrane bones, i.e. ossifications in membrane 

 without any cartilaginous precursors. 



The names which have been given to the various 

 parts of the cartilaginous cranium in the above account 

 are derived from the names given to the bones appear- 

 ing in the respective regions in the more developed 

 skull. 



The skeleton of the visceral arches. The visceral 

 arches were all originally branchial in function. They 

 supported the walls between successive branchial clefts. 



The first arch (mandibular) has in all living forms 

 lost its branchial function, and its bar has become con- 

 verted into a supporting skeleton for the jaws. 



