Skull and Hyoid- 265 



\\V find, then, that the skull can be said to be made up of bones which may be 

 divided into class- s according to their source of origin. Thus we have : 



(a) Bones of the Cranial Capsule. 



(b) Special Sense Capsules. The ethmoid and petrous in this group can perhaps 

 be added as a subdivision to (a), but the vomer, nasal, and lachrymal may be placed 

 in this class as formed in membrane on the capsule, and the premaxilla may perhaps 

 be put provisionally with them. 



(c) Visceral Skeleton. This includes (i) membrane bones formed in the arches, 

 mandible, maxilla, malar, palate, internal pterygoid plate ; (2) bones formed in cartila- 

 ginous bars of the arches styloid process, ossicles of ear. 



(d) The Tympanic Plate. The value of this is uncertain, and, although it is often 

 referred to as homologous with certain ossifications found in lower types of jaw, it would 

 be safer from our present point of view to place it simply in a class by itself without 

 further remark on its morphology. 



The bones of the cranial capsule can be further subdivided into cartilage and 

 membrane bones : the membrane bones are the frontal, parietal, squamous temporal, 

 and interparietal part of occipital, with a large part of the great wing of sphenoid. 

 The chondro cranium underlies that part of the brain that expands least, and the 

 membrane bones cover the portion that grows most rapidly. 



The cerebral nerves pass out of the embryonic skull between the various parts of the chondro- 

 cranium, and the foramina of exit in the adult skull may be divided into groups on this basis. 

 Thus we get foramina (a) between various distinct parts of the skull, as the jugular between petrous 

 and occipital ; (b) between different parts of what is one bone (in the adult), as the anterior condy- 

 loid foramen between basi- and ex- occipital. A class occurs as a subdivision of (a), the foramen 

 being formed by ossification extending from one of the bordering elements round the issuing struc- 

 ture : thus we may get the foramen ovale, for instance, as a secondary enclosure of a nerve originally 

 passing between alisphenoid and periotic capsule, or in some cases, as in the canalis innominatus 

 of the sphenoid, the enclosure may only take place occasionally. 



We have seen that the skull can be broadly divided into a dorsal and lateral group 

 of bones developed in relation with the capsule of the brain, and a ventral skeleton 

 of a visceral origin. 



So far the division, given above, is founded simply on the embryonic develop- 

 ment and is therefore unassailable. But when we come to inquire into the remoter 

 values of the parts concerned, and to compare the human skull with those of lower 

 types, the situation becomes more complicated and bristles with difficulties. 



So far as the mammals are concerned, the comparison is easy and straightforward, but below 

 this class the difficulties begin. In all classes of vertebrate skulls the uncertainties are not so much 

 with regard to the bones of the brain capsule, nor with reference to the greater part of the visceral 

 skeleton but are mainly associated with the comparative values of the prechordal and nasal 

 capsular regions in fact, they may in general be said to centre round the nose and roof of the 

 mouth. 



It would be out of place in the present work to enter on a consideration of the questions in- 

 volved, and it will be sufficient to mention that the membrane bones are generally thought to 

 represent dermal structures which, originally placed superficial to the coverings of the brain, as 

 this expanded in evolution came to occupy a deeper situation to fill the call for increased covering : 

 thus in the Elasmobranch fishes the brain case is a continuous cartilaginous one, but as we proceed 

 further up the vertebrate scale we find the cartilage becoming confined to a relatively lessening 

 basal area, while its place on the vertex is taken by membrane bones presumably derived from more 

 superficial tissues. A similar view is taken of the formation of the jaws, but here the question 

 is complicated by the failure of the human bones to show centres of ossification corresponding 



