VISCERAL SKELETON 



257 



broad process projects forwards to be attached to the roof of the nasal capsule. 

 In later stages this is largely reabsorbed. The ethmoidal region of the human 

 skull is peculiar in respect that the nasal capsules are so rotated that the 

 apertures of communication between the cranial cavity and the nasal fossae are 

 placed horizontally. Each aperture is at first single, but is afterwards converted 

 into the cribriform plate by the formation of cartilaginous bridges. The central 

 cartilage is continued forwards as the septum nasi, and its upper edge projects 

 between the olfactory openings as the crista galli. The side walls of the nasal 

 capsules bend inwards in front to be continuous with the edge of the septum and so 

 complete the roof of the nasal fossae, but the floor is deficient, and only completed 

 when the palatal processes have met with the lower free edge of the septum. From 

 the mesial aspect of the outer wall of the nasal capsule on either side project the 

 cartilaginous rudiments of the several turbinate processes. 



for. optic. ala orbital i 

 \ 



palat. 



dentale 



cart, cricoid. 



cart, tfiyreoid. 



for. hypogl. 



FIG. 310. THE SAME MODEL AS SHOWN IN FIG. 309, FBOM THE LEFT SIDE. 

 Certain of the membrane-bones of the right side are represented in yellow. 



The visceral skeleton consists of a number of skeletal elements which occupy 

 the mandibular, the hyoid, and first branchial arches. In the mandibular arch 

 a bar of cartilage is laid down, known as MeckeTs cartilage. This represents the 

 primitive mandible ; but it in great part disappears, being displaced by a membrane 

 bone, the os dentale. A small portion of its ventral end, however, directly ossifies 

 and forms a part of the lower jaw, and its proximal end is developed into the malleus. 

 Close to this a separate formation in the blastema in the base of the arch gives 

 rise to the incus. In the hyoid arch are formed the stapes, the styloid process of 

 the temporal bone, the stylohyoid ligament, and the lesser cornu of the hyoid bone ; 

 while in the first branchial arch a short bar of cartilage is deposited which becomes 

 the greater cornu of that bone. The body of the hyoid bone is an intermediate 

 formation between the ventral ends of the second and third arches. The develop- 

 ment of the visceral skeleton is specially interesting as providing the proof 

 that proximal portions of the first two arches, which in lower vertebrates 

 VOL. i. s 



