THE CRANIUM. 169 



extremities fuse round the anterior termination of the notochord 

 with the parachordal cartilages. The buccal part of the pituitary 

 grows into the cranial cavity in front of the notochord and 

 keeps the two cartilages apart ; but in front of the pituitary 

 the two bars fuse in the middle line. The mesial fused 

 parts of the trabeculae grow into the mesial nasal processes 

 of which they form the skeletal basis and become transformed 

 into the primitive cartilaginous septum of the nasal cavities 

 (Figs. 136 and 3, p. 3). The posterior segment of the median 

 fused bars forms the cartilaginous basis of the pre-sphenoid and 

 basi-sphenoid (Fig. 136). From the trabeculae four lateral 

 processes or wings grow out on each side (Fig. 136). The pos- 

 terior, which is small at first, forms the great wing of the 

 sphenoid and external pterygoid plate; the second is originally large, 

 and forms the small wings (orbito-sphenoids) ; the third and fourth 

 outgrowths are closely joined, — they form the lateral masses of 

 the ethmoid and alar cartilages of the nose (Fig. 6, page 6). The 

 nasal bones, the lachrymal and ascending nasal processes of the 

 superior maxilla, develop in the membrane over the lateral nasal 

 wings of the trabeculae, in the same way as the vomer develops 

 over the cartilage of the septum. 



Development of the Sphenoid (Fig. 137). — At birth the 

 sphenoid bone, which is developed by ossification of the posterior 



<rbito-sph. 

 presphenoid/ ^ opthfon 



ali-sphen. 



for. rot. 



'. pteryg. proc. 



for. ovale 



int. pter. bas '- s P h 



Fig. 137 —The Sphenoid in a foetus of 4 months. The Centres of Ossification are 

 deeply shaded. (After Sappey.) 



parts of the trabeculae cranii, consists of three parts, the 

 great wings being separated from the rest of the bone. The 

 sphenoidal turbinate bones, afterwards inflated by the development 



