848 DEVELOPMENT [CH. LIX. 



nasal part of the stomadseal space forming the rudiments of the 

 ethmoid and inferior turbinal bones, and a mesial process descends into 

 a septum which has grown down from the under surface of the fronto- 

 nasal process, and united with the palate dividing the nasal chamber 

 into right and left halves. In this the vertical plate of the ethmoid 

 and the vomer are ossified. The posterior parts of the trabeculte fuse 

 with the basilar plate and form the rudiment of the presphenoid. 

 Posteriorly, and at the sides, cartilaginous plates grow over the cerebral 

 vesicles; but in mammals the occipital region alone is roofed in by 

 cartilage; the rest of the cranial vault being formed of membrane 

 bones. 



From the sides of the presphenoid, the lesser wings or orbito- 

 sphenoids containing the optic foramina are developed, and from the 

 sides of the basi-sphenoid the greater wings or alisphenoids. A 

 cartilaginous capsule invests the auditory vesicle, and becomes con- 

 nected to the parachordal cartilage on each side. It is called the 

 periotic capsule ; it is replaced by bone, which constitutes the petrous 

 and mastoid portions of the temporal bone. 



Cartilaginous bars appear in the visceral arches, and from that in 

 the mandibular arch on each side Meckel's cartilage; the symphysis 

 of the jaw, the malleus, and possibly the incus are formed. The 

 stapes is the result of a separate ossification round the stapedial 

 artery. The remainder of the mandible is ossified in the membrane 

 around the mandibular cartilage. 



From the second bars the anterior part of the body of the hyoid 

 bone, its small cornua, the stylo-hyoid ligaments, and the styloid pro- 

 cesses are developed. The cartilages of the third arches give rise to 

 the posterior part of the body of the hyoid, and its great cornua; 

 the cartilages of the remaining arches take part in the formation of 

 the cartilages of the larynx. 



In mammals the clefts between the arches are merely grooves 

 which do not communicate with the cavity of the foregut, as they do 

 in fishes and amphibians. The outer depression of the first cleft forms 

 the external auditory meatus, and the inner depression is converted 

 into the tympanic cavity and the Eustachian tube. The remaining 

 clefts disappear. 



The cranial nerves are also associated with the arches and clefts. 

 The third division of the fifth is distributed to the mandibular arch, 

 its second division goes to the maxillary process, and its first division 

 to the fronto-nasal process. The seventh is the nerve of the second 

 arch, the ninth belongs to the third arch, and the remaining arches 

 are associated with the tenth nerve. 



