126 THE SKULL [chap. 



The third, or first post-oral arch (HI.), called the first, 

 in the older and more usual nomenclature, consists of 

 Meckel's cartilage, a slender rod, in relation above with the 

 periotic region of the skull. Of this, in the Mammalia, the 

 upper extremity becomes converted into the malleus, one 

 of the small bones in the tympanic cavity, while in con- 

 nection with the lower or distal part, the ramus of the man- 

 dible or lower jaw is developed partly by conversion of the 

 cartilage itself, but principally by the ossification of fibrous 

 or cartilaginous tissue deposited around it. The upper end 

 of the ramus afterwards acquires a secondary articular con- 

 nection with the squamosal bone, its primitive connection 

 with the malleus entirely disappearing. 



The cleft which lies behind this arch becomes contracted 

 into the Eustachian tube, tympanic cavity, and meatus 

 auditorius externus, which would form a canal of commu- 

 nication between the pharynx and the external surface but 

 for the interposition of the delicate inembrana tympani. 



The rod of cartilage forming the fourth visceral arch 

 (IV.), or second post-oral, becomes the anterior hyoid arch, 

 its proximal extremity being modified into the incus} 



From the fifth arch (third post-oral), which corresponds 

 to the first branchial of branchiate vertebrates, is formed 

 the posterior hyoid arch, or thyrohyal. (V.) 



Some of the changes which take place in the cranium while 

 advancing from youth to maturity have already been noticed ; 

 but it will be well, before proceeding to describe the modi- 

 fications of the mammalian skull, to mention certain others 

 Avhich take place, to a greater or less degree, in all skulls. 



^ The stapes, according to Parker, is formed independently of the 

 visceral arches, in a budding of the outer cartilaginous wall of the 

 auditory bulla. 



