DEVELOPMENT OF THE CRANIUM. 



733 



back of which the vomer is formed as a bony splent covering ; while in 

 the hinder lyre-shaped interval of the separated trabeculge is placed the 

 infundibulum in connection with the pituitary body. 



Fig. 532. 



Fig. 532. — Basilar Part op the Primordial Cranium 

 OF A Human Fcetus of three months seen from 

 ABOVE (from Kolliker). 



a, iipi)cr half of the squama occipitis ; h, lower half of 

 the same ; c, cai-tilaginous plate extending into it ; d, 

 (in the foramen magnum) the exoccipital ; c, basi-occipital ; 

 /, peti'ous, with the meatus anditorius interuus ; g, dorsum 

 selliB, with two nuclei belonging to the basi-sphenoid 

 bone ; /;, nuclei in the anterior clinoid processes ; i, great 

 wing nearly entirely ossified ; I; small wings ; I, crista 

 galli ; m, cribrethmoid ; n, cartilaginous nose ; o, strip of 

 cartilage between the sphenoid and the parietal ; p, 

 osseous plate between the lesser wings and the cribri- 

 form plate. 



From the side of the presphenoid cartilage the matrix of the orbito- 

 sphenoids or lesser wings, containing the optic foramina, is developed ; 

 and from the sides of the basi-sphenoid proceeds the matrix of the 

 greater wings, which are also cartilaginous in their origin. 



In the periotic or cartilaginous rudiment of the temporal bone three 

 centres of formation are distinguished by Huxley, viz., 1. OpisthoUc, or 

 that surrounding the fenestra rotunda and cochlea ; 2, prootic, or that 

 which encloses the superior semicircular canal ; and 3, epiotic, or that 

 which surrounds the posterior semicircular canal and extends into the 

 mastoid portion. They soon unite into one so as to form the petro- 

 mastoid bone. 



Fig. 533. — Longitudinal Section through the 

 Head op an Embryo op Four Weeks (from 

 Kolliker). -j"- 



V, anterior encephalic vesicle, cerebral portion ; 

 z, interbrain ; m, midbrain ; h, cerebellum ; n, 

 medulla oblongata ; no and a, optic vesicle ; 

 o, auditory depression ; t, centre of basi-cranial 

 flexure ; t', lateral and hinder parts of tentorium ; 

 p, the fold of epiblast which forms the hypo- 

 physis cerebri. 



The styloid process and the auditory 

 ossicles are of cartilaginous origin. 



The squamo-zygomatic and tympanic are produced from membrane. 



Tlie Cranial Flexures. — The earliest and the most important 

 of the cranial flexures is that which takes place at the anterior 

 extremity of the notochord and in the region of the mid-brain or 

 middle encephalic vesicle. Here, as previously stated, the notochord 

 extends into the substance of the basis of the cranium as it is prolonged 

 forwards in the line of the vertebral bodies. At this place the medul- 

 lary tube, and the substance forming the wall of the cranium especially, 

 undergoes a sudden bending downwards and forwards, so as to cause the 

 projection of the thickened cranial base in a marked manner upwards. 

 This coincides with the place where the investing mass and the trabe- 



