OSSIFICATION OF THE BONES OF THE HEAD. 



75 



forming the upper and fore part of the mastoid division of the bone. Beneath this is an air- 

 space continued backwards from the tympanum, and called the antruin mastoidcum, from 

 which the mastoid cells subsequently grow out. During the third month an osseous nucleus 

 appears in the lower part of the external membranous wall of the tympanum, and extends 

 upwards forming the tympanic ring, an imperfect circle, open above, which encloses the 

 tympanic membrane. Before birth the extremities of this ring become united with the 

 squamo-zygomatic. 



Petro-mastoid or periotie. It is only in the latter half of the fifth month that osseous 

 deposits begin to be formed in the cartilaginous ear-capsule. They extend rapidly, and the 

 different ossifications are united by the end of the sixth month. The first to appear is a 

 nucleus on the promontory (ppisthotic), which spreads round the fenestra rotunda, and forms 

 the portion of the petrous below the internal auditory meatus and fenestra ovalis. The second 

 (prootic} arises over the superior semicircular canal, and forms most of the petrous seen in the 

 interior of the skull, as well as the upper and inner part of the mastoid : it furnishes the upper 

 boundaries of the internal auditory meatus and fenestra ovalis. A little later, a third nucleus 



Fig. 77. SEPARATE PARTS OP THE TEMPORAL BONE OP A FCETUS 



SHORTLY BEFORE BIRTH. (R. Quain.) 



a, squamo-zygomatic ; b, tympanic ; c, mastoid part of periotie ; 

 d, inner wall of tympanum ; e, mastoid antrum. 



(epiotie), which is occasionally double, is developed in con- 

 nection with the posterior semicircular canal, and extends 

 into the lower part of the mastoid. According to Sutton 

 the tegmen tympani and covering of the external semi- 

 circular canal are formed by a separate ossification (pterotic), 

 appearing about the same time as the prootic. Vrolik also 

 described a special nucleus for the roof of the cochlea, and 

 another in connection with the common crus of the superior 

 and posterior semicircular canals. (Huxley, " Lectures on 



Comparative Anatomy," 1864 ; Vrolik, Niederl. Arch. f. Zoologie, i, 291 ; Sutton, Journ. 

 Anat., xvii, 498.) 



At birth the petro-mastoid is separated from the squamosal by a thin plate of intervening 

 cartilage, bony union taking place during the first year ; the mastoid portion also is flat, the 

 glenoid fossa shallow, the articular eminence scarcely to be seen, and the tympanic ring and 

 membrane are even with the outer surface of the bone. The anterior and inferior walls of 

 the external auditory meatus consist at first of fibrous tissue, in which the tympanic plate is 

 formed after birth. 1 The latter is developed from the outer margin of the slender tympanic 

 ring, commencing in the form of two small tubercles at the fore and hinder parts respectively ; 

 these increase in size and meet in the floor of the meatus, enclosing a foramen which is 

 gradually closed. The foramen is completed as a rule in the second year, and is seldom 

 obliterated before five years of age. The part of the wall of the meatus which was occupied 

 by the foramen is commonly thin, and sometimes a small aperture persists through life. 

 On the posterior surface of the petrous at birth is a considerable depression which extends into 

 the arch of the superior semicircular canal, and represents the floccular fossa of the lower 

 animals ; in the adult bone a vestige of this is always present as a small fissure above and 

 outside the internal auditory meatus, between that and the aqueduct of the vestibule. The 

 mastoid process is developed about the second year, but the air-cells are not formed till near 

 puberty. 



The styloid process is formed by two small ossifications in cartilage the tympanohyal 

 at the base, commencing before birth and speedily joining the bone, and the stylohyal 

 commencing usually after birth, but remaining very small until the period of puberty ; this 

 only joins after adult age is reached, and often remains permanently separate. (Flower, Brit. 

 Ass. Rep., 1870.) 



The sphenoid bone is divided in the foetus into a posterior or postsphenoid part, to which 

 the sella turcica and great wings belong, and an anterior or presphenoid part, including the 

 body in front of the olivary eminence and the small wings, a division which is found in 

 many animals persistent through life. The first osseous nuclei of the postsphenoid division 

 appear about the eighth week in the great wings (alisphenoids), between the foramen 

 rotundum and foramen ovale, and spread thence outwards into the wing and downwards into 

 the external pterygoid plate. About the same time also two granules appear in the post- 

 sphenoid part of the body (basisphenoid), placed side by side in the sella turcica ; these units 



. J See J. Symington, Jouru. Anat., xix, 284, and "The Topographical Anatomy of the Child," 



1887, P. 46. t :..- 



