582 MR. J. B. SUTTON ON THE DEVELOPMENT [JunC 2, 



Later a nncleus makes its appearance on the inner side of each 

 optic foramen, on the deep aspect of the perichondrium : these are 

 the presi)henoidal centres. They attain some considerable size before 

 involving the cartilage, and can during the first month of their 

 existence be removed without in any way disturbing the subjacent 

 cartilage. The orbitosphenoid quickly sends down two spurs around 

 the optic nerve, which fuse with the presphenoid. The presphenoids 

 in their turn send a thin shell of bone across the dorsal aspect of the 

 cartilage to fuse with each other ; a small circular spot of cartilage 

 long remains to indicate the point around which they united. It is 

 long before these presphenoidal nuclei fuse below ; a large piece of 

 cartilage, belonging to the etbmo-vomerine plate, separates them, 

 even for some months after birth. Long before this occurs the 

 presphenoids, bearing their allies the orbito-sphenoids, have fused 

 with the basisphenoids ; the line of fusion being represented in 

 afterlife by the ridge known as the olivary process. 



Before dismissing the orhito-sphenoid, it is necessary to draw 

 attention to one circumstance connected with it, of some interest. 

 The lateral extension of the traheculse to form the side-wall of the 

 chrondro-cranium is in the later stages replaced almost entirely by 

 the alisphenoids. 



If the region of the side-wall of the skull known as the anterior 

 lateral fontanelle in the foetus (in the adult it is called the pterion) 

 be examined between the fourth and seventh months of intra-uterine 

 life, it will be easily noted that the cartilaginous orbito-sphenoid 

 extends into this fontanelle, so that for a considerable period it helps 

 to form the side-wall of the skull. In man the permanent orbito- 

 sphenoid never extends so far outwards as its cartilaginous forerunner, 

 leaving the space to be filled in by the epipteric bone. The details 

 of the ossification of this region I have considered elsewhere '. 



There remains little to add concerning the later development of 

 the orbito-sphenoid, except to note that eventually the orbito- 

 sphenoids of opposite sides send a thin lamella across that portion 

 of the presphenoid which is anterior to the optic groove, thus 

 excluding it from the cranial cavity. 



The portions of the sphenoid previously considered are strictly 

 cranial, but it receives an additional element from one of the 

 appendages of the skull, viz. the palato-pierygoid bar. 



In a paper published in the Proceedings of this Society for 

 1884, " On the Parasphenoid " &c., facts were adduced to show that 

 the anterior portion of the palato-pterygoid cartilage in man became 

 ossified to form the internal pterygoid plate ; the nucleus for this bone 

 may be detected as early as the commencement of the third month 

 of intra-uterine life. The length of time it may remain as a separate 

 ossicle varies within wide limits. I have seen it distinct from 

 the sphenoid as late as the fifth month ; but as a rule it will be found 

 united with the under surface of the alisphenoid at the commencement 

 of the fourth, so that it joins the alisphenoid before that bone 



^ " On tlie Eelation of the Orbito-spbonoid to the Pterion," Journal of 

 Anatomy and Physiology, vol. xviii. p. 219. 



