242 MAMMALIAN ANATOMY 



VARIATIONS IN FORM AND DEVELOPMENT. 



Beyond slight fluctuations in the relative prominence of the parts, 

 the temporal bone presents few noteworthy variations. 



It is said that the postglenoid foramen is absent in the cat ; 

 an examination of one hundred temporals, taken at random from a 

 large collection, showed, however, that it was present in all but three. 

 It varies somewhat in position, and is often hidden by the edge of the 

 auditory bulla. In twenty-nine specimens it was at the medial side 

 of the postcondyloid process ; in nine of these it was accompanied 

 by a second foramen, which was sometimes very large, situated at 

 a distance dorsally and laterally on the surface of the zygoma. Its 

 usual position is behind the inner end of the root of the process. The 

 postcondyloid foramen may be double ; the two foramina are then 

 close together side by side, or one is in front of the other (three 

 specimens) ; in one specimen there were three foramina. A small 

 accessory foramen is sometimes to be seen in the fossa for articulation 

 Avith the ectotympanic part of the bulla. 



In one specimen a well-marked foramen was observed in the 

 posterior root of the zygoma where it joined the lambdoidal ridge. 

 In two specimens a foramen entered the posterior part of the outer 

 surface of the squamous, midway between the zygomatic root and the 

 superior border ; in another a small foramen pierced the same surface, 

 near the superior anterior border. 



In the petrous portion of the bone, the superior border may be 

 elevated in its basal part into a crest overhanging a decided groove 

 for the superior petrosal sinus, which curves downward and backward 

 to the groove for the lateral sinus. In its apical part the border may 

 be deeply excavated for the fifth cranial nerve. 



In some specimens the appendicular fossa in the posterior surface is 

 very shallow, in others it is as deep as the auditory meatus and pierced 

 at the bottom by a small foramen. The swelling for the superior semi- 

 circular canal may be abnormally high or may be entirely absent. In 

 some specimens the position of the posterior semicircular canal is 

 clearly outlined by a longitudinal elevation. The shape of the in- 

 ternal auditory meatus varies from oblong to round ; the division for 

 the facial nerve may be as large as the division for the auditory nerve. 

 In many specimens the groove for the inferior petrosal sinus along 

 the inferior border, as well as the opening for the aquseductus cochleae,. 



