338 



THE CAT. 



[CHAP. x. 



oundation for the lower jaw, and ends by uniting with its fellow of 

 the opposite side. 



The next, or second cartilaginous rod (//), connected above, like 

 the first, with the auditory capsule, descends in the second visceral 

 arch, and lays the foundation of the anterior cornu and body of the 

 os hyoides. 



The cartilage of the next visceral arch (Fig. 154, th] is the pre- 

 cursor of the posterior cornu of the os hyoides. No noticeable 

 permanent part of the skeleton is formed in the fourth visceral arch. 



10. The process of OSSIFICATION of the skull begins with the 

 mandible, where one centre of ossification appears in each lateral 

 moiety at an earlier period than in any other part of the skeleton. 

 It is an ossification in the membrane investing Meckel's cartilage, 

 and not in the cartilage itself. 



Immediately after the mandibular ossification, follows that of the 

 maxillaa and premaxilla}. 



The premaxilla) are ossifications in the cartilage of the naso- 

 frontal process, while the maxillao are ossifications in the membrane 

 investing the maxillary processes. The palatine and pterygoid 

 bones arise as ossifications in the cartilage of the maxillary 

 process. 



Fresh points of ossification also indicate the supra-occipital, the 

 parietak and interparietak, the f rentals, the squamosals, the vomer, 

 the nasals, the lachrymals, and the malars, all these bones being 

 ossified directly from membrane, and not through pre-existing 

 cartilage ; but cartilaginous ossifications also indicate the future 

 basi- and ex-occipitals, the all-sphenoids, the basi-sphenoids, and the 

 orbito-sphenoids, with the pre-sphenoid, as also the palatines and 

 pterygoids, as above mentioned. Other ossifications arise successively 

 and at various intervals. Thus, the median ethmoid and cribriform 

 plate ossify late, as also do the bones of the os hyoides. The vomer 

 is an ossification of the membrane investing laterally the lowest part 

 of the ethmo-vomerine cartilage. 



The occipital bone for a time exists in four distinct pieces, the 

 basi-, ex- and supra- occipitals. 



The frontal bones remain divided from one another by a con- 

 tinuation forwards of the sagittal suture. 



The sphenoid bone consists at an early period of eight distinct 

 and significant parts : the bulk of the body, or basi-sphenoid ; the 

 front part of the body, or pre-sphenoid ; the greater wings, or ali- 

 sphenoids ; the lesser wings, or orbito-sphenoids ; and the true 

 pterygoid bones. 



The temporal bone arises from many distinct centres, and consists 

 for a time of several distinct bones. Amongst these bones are the 

 squamosal (including the squamous part and zygomatic process), the 

 tympanic parts, and the tympano-hyal. The bone which forms 

 the inner and larger chamber of the tympanic cavity does not 

 appear till a fortnight after birth. It sends in a process to form the 

 septum, as does also the outer tympanic bone, so that the septum in 



