292 OSTEOLOGY. 



The region occupied by the ala temporalis is slow to chondrify. According to Fawcett, the 

 only part of it which is preformed in cartilage is that which corresponds to the root of the two 

 pterygoid laminae in the adult : this is, perforated by the maxillary division of the trigeminal 

 nerve. According to the same authority, the whole of the lateral pterygoid lamina and that 

 part of the ala temporalis projected into the orbital and temporal fossae are ossified in membrane. 

 So, too, are the foramen ovale and foramen spinosum. 



The ala orbitalis, at first much larger than the ala temporalis, is described as chondrifying in 

 the following way. The process begins by the appearance of cartilage posterior to the position 

 of the optic foramen ; medially this fuses with the lateral aspect of the anterior part of the body 

 of the sphenoid, laterally it extends into the orbital plate, with the independent cartilaginous 

 centre of which it unites. The foramen opticum is completed by the extension of the cartilage 

 from the side of the anterior extremity of the body of the sphenoid, in front of the nerve, to 

 reach the orbital plate. These three centres fuse to form a single piece of cartilage during the 

 third month. 



Anterior to the orbito-sphenoids, the base of the skull is intimately associated with the nasal 

 capsule, and is the last part of the chondro-cranium to become cartilaginous, this change not 

 being effected till the third month. The roof of the capsule is formed by the coalescence of 

 cartilaginous elements appearing, first in the nasal septum by an extension of the cartilage from 

 the ventral surface of the body of the sphenoid and secondly by an independent centre in each 

 lateral wall of the capsule. At first the nasal capsule is open dorsally on either side of the nasal 

 septum in correspondence with the olfactory bulbs, but during the third month the wall of the 

 capsule corresponding to the cribriform plate commences to chondrify around the perforating 

 nerve-fibres, and so the lamina cribrosa is preformed in cartilage. Laterally strips of cartilage 

 (cartilago ethmosphenoidalis) pass backwards from the lateral edges of the cribriform plate to 

 unite it with the anterior edges of the alae orbitales of the sphenoid. 



By the third month the nasal capsule has become cartilaginous. As has been stated above, 

 the nasal septum chondrifies by an extension forwards of the ventral part of the body of 

 the cartilaginous sphenoid. On either side of the ventral margin of this septum anteriorly are 

 developed the paraseptal cartilages, which in man persist till after birth. These are connected 

 posteriorly by means of a connective tissue bridge with small pieces of cartilage the posterior 

 paraseptal cartilages, which are in turn associated with the paranasal cartilages posteriorly, 

 and there in part form the floor of the recessus terminalis or cupola of the cartilaginous nasal 

 capsule (Fawcett). In man, owing to the deficiency of the lamina transversalis anterior, the 

 fenestra narina and the fenestra basalis which pierce the floor of the nasal capsule, on either 

 side of the septum, become confluent and form the fissura rostroventralis of Gaupp. Meanwhile 

 the lateral walls of the nasal capsule are chondrifying independently, forming the paranasal 

 cartilages. These become subsequently united anteriorly with the nasal septum to form the 

 tectum nasi or roof of the nose. At first this is open posteriorly where it is in relation with 

 the olfactory bulb, but later, as has been already described, the tissue around the nerve filaments 

 chondrifies to form the cartilaginous lamina cribrosa. The inferior concha is derived from the 

 cartilage of the lower and lateral part of the nasal capsule, from which, however, it becomes 

 isolated about the seventh month. Above and behind this the middle and superior conchae, 

 the ethmoidal turbinals, become chondrified, as well as the cartilaginous rudiments which 

 subsequently form the agger nasi, the bulla ethmoidalis, and the concha sphenoidale or 

 ossiculum Bertini. 



Throughout life certain parts of the cartilaginous nasal capsule persist as the cartilaginous 

 nasal septum and the cartilages of the alae of the nose, whilst other parts are absorbed and 

 are replaced by surrounding bones of membranous origin. 



The various foramina met with in the cranial base are formed either as clefts in the line of 

 union of the several cartilaginous elements, or through inclusion by means of bridging processes 

 derived from these same elements. 



From the ventral surface of this cartilaginous platfornr formed, as described, by the union 

 of the trabeculse, parachordal cartilages, and cartilaginous auditory capsules is suspended the 

 cartilaginous framework of the visceral arches, which play so important a part in the develop- 

 ment of the face, an account of which is elsewhere given. 



A consideration of the facts of comparative anatomy and embryology appears to justify the 

 assumption that the mammalian skull is of twofold origin that, in fact, it is composed of two 

 envelopes, an outer and an inner, primarily distinct, but which in the process of evolution have 

 become intimately fused together. The inner, called the primordial skull, is that which has 

 just been described, and consists of the choiidro-cranium and the branchial skeleton. The outer, 

 which is of dermic origin, includes the bones of the cranial vault and face which are developed 

 in membrane. This secondary skull, which first appears in higher fishes as ossified dermal plates 

 overlying the primary skull, acquires a great importance in the mammalia, as owing to the 

 expansion of the brain and the progressive reduction of the chondro-cranium, these dermal bones 

 become engrafted on and incorporated with the primordial skull, and act as covering bones to 

 the cavities of the cranium and face ; for it may be well to point out that these dermal or 

 membrane bones are not necessarily external in position, as over the cranial vault, but also 

 develop in the tissues underlying the mucous membrane of the cavities of the face. 



Advantage is taken of this difference in the mode of development of the bones of the skull to 

 classify them according to their origin into cartilage or primordial bones, and membrane or 

 secondary bones. These differences in the growth of the bone must not be too much insisted 

 on in determining the homologies of the bones of the skull, as it is now generally recognised that 



