290 



OSTEOLOGY. 



APPENDIX E. 



DEVELOPMENT OF THE CHONDRO-CRANIUM AND MORPHOLOGY OF 



THE SKULL. 



As has been already stated, the chorda dorsalis or notochord extends headwards to a point 

 immediately beneath the anterior end of the mid-brain. In front of this the head takes a bend 

 so that the large fore-brain overlaps the anterior extremity of the notochord. At this stage of 

 development the cerebral vesicles are enclosed in a membranous covering derived from the mesen- 





Crista Galli 



Pars ethmoidalis 



Lamina cribrosa 



Orbito-sphenoid 



Superior orbital fissure 



Alisphenoid 



Carotid canal 



Meatus acusticus 

 internus 



Subarcuate fossa- 

 Jugular foramen 



Canalis hypoglossi 



Foramen magnum 



Orbital portion of orbito-sphenoid 



t Optic foramen 



Tuberculum sell* 

 (Olivary process) 



[ Sella turcica 

 Dorsum sellae 

 7- Pars petrosa 



Superior semicircular cam 



Pars mastoidea 



Supra-occipital 



Occipital fontanelle 



FIG. 282. VIEW OF THE CHONDRO-CRANIUM OP A HUMAN FOETUS 5 CM. IN LENGTH FROM VERTEX 

 COCCYX (about the middle of the third month) ; the cartilage is coloured blue. The line to the rig 

 of the drawing shows the actual size. 



chyme surrounding the notochord ; this differentiated mesodermal layer is called the primordic 

 membranous cranium. From it the meninges which invest the brain are derived. In lowei 

 vertebrates this membranous capsule becomes converted into a thick -walled cartilaginous envelope, 

 the primordial cartilaginous cranium. In mammals, however, only the basal part of this capsule 

 becomes chondrified, the roof and part of the sides remaining membranous. In considering the 

 chondrification of the skull in mammals, it must be noted that part only of the base is traversed by 

 the notochord, viz., that portion which extends from the foramen magnum to the dorsum sellae of the 

 sphenoid. It is, therefore, conveniently divided into two parts one posterior, surrounding the 

 notochord, and hence called chordal, and one in front, into which the notochord does not extend, 

 and hence termed prechordal. These correspond respectively to the vertebral and evertebral 

 regions of Gegenbauer. In the generalised type, a pair of elongated cartilages called the para- 

 chordal cartilages appear on either side of the chorda in the chordal region, similarly in the 

 prechordal region two curved strips of cartilage named the prechordal cartilages, or the 

 trabeculae cranii of Rathke, develop on either side of the cranio-pharyngeal canal. In the 

 human embryo, however, this symmetrical arrangement has not hitherto been observed. In 

 man, chondrification of the cranial base commences early in the second month and attains its 

 maximum development about the end of the third month, at which time the chordal part of 

 the chondrocranium consists of a ring of cartilage, the ventral part of which is formed by the 

 fusion of two parachordal cartilages, so forming around the chorda dorsalis, a central axial part, 



