THE BONES OF THE HEAD 



159 



form being known as plagioeephalas (oblique or awry head). \\Tien the 

 metopic or frontal suture becomes prematurely obUterated, growth is arrested 

 in the frontal region, and the skull assumes a triangular shape. This variety 

 is known as trigonocephalas. When premature obliteration of the sutures at 

 the base of the skull takes place, the deformity known as cretin skull results. 

 This is characterized by enlargement of the cranium (except in the occipital 

 region), which becomes very hea\y, and assumes an irregular, somewhat 

 conical shape, with the apex at the sagittal suture. It is associated with 

 mental dulness, idiocy, and stunted growth, and the general condition is 

 known as cretinism. 



Development of the Skull. 



The skuU is composed of two divisions, which are genetically distinct from 

 each other. One division represents the cranium, which is developed from the 

 mesenchyme (mesoderm) . around the cephalic part of the notochord. The other 

 division, which includes the faca and the hyoid bone, represents the Tlsceral- 



FiG. 89. — Diagrams of the Primitive Cartilaginous Cranium 

 (Wiedersheim). 



A. First Stage. — N, Notochord; PchC, Parachordal Cartilage; Tr, Prechordal 



Cartilage; PPS, -Primitive Pituitary Space; Olf, Opt, Aud, positions of 

 Organs of Smell, Sight, and Hearing. 



B. Second Stage. — N, Notochord ; PchR, Parachordal Region (Basilar Plate) ; 



TrR, Fusion of Prechordal Cartilages; PPS, Primitive Pituitary Space; 

 TrR, Prechordal Region; Olf, Opt, Aud, as in A. 



arch skeleton, and is developed, for the most part, from the first, second, and 

 third visceral arches. The stages in the development of the skull £ire, as in 

 the vertebral column, membranous, chondral or cartilaginous, and osseous. 



The Cranium. 



The cranium, as jiist stated, is developed from the mesenchjTne which sur- 

 rounds the cephalic part of the notochord. 



This chord, after passing through the future odontoid process of the axis, 

 extends forwards to the posterior part of the pituitary region of the base of 

 the cranium, where the dorsum sellae of the sphenoid bone is subsequently 

 developed. In this situation it Ues on the ventral aspect of the mid-brain, 

 and within the sharp ventral bend described by the fore-brain. The cephalic 

 part of the notochord carries along with it the skeletogenous sheath of mesen- 

 chyme. 



