CRANIUM. 



733 



which takes place beween the several elements 

 of the bones of the cranium, that Soemmering 

 and Meckel have described the two as one 

 bone, under the name of os basilare or osspheno- 

 occipitale. 



The connexions of this bone are few and 

 simple, being, in its superior half, with the 

 parietals ; in its inferior half, with the tem- 

 porals ; at its anterior extremity, with the 

 sphenoid; and, by its condyles, with the 

 atlas. 



At birth this bone is separable into four dis- 

 tinct portions, one being in front, one behind, 

 and one on each side of the foramen magnum, 

 the border of which is, consequently, not then 

 completed. The anterior and two lateral por- 

 tions are formed by the extension of ossific 

 matter from one point of deposit in each ; but 

 that posterior to the foramen is produced from 

 many points, in the number of which ana- 

 tomists are not agreed. The ossification com- 

 mences in the lower part, at some distance 

 from the foramen, by one point on each side 

 of the median line ; and before they have 

 completely approached each other, two ana- 

 logous deposits appear in the upper part, which 

 coalesce before the upper and lower pieces are 

 joined. This occurs during the fourth month, 

 at which time the inferior and broad part dis- 

 plays on each side another point of ossification 

 on a level with the spot where the process 

 first commenced; in the fifth month the whole 

 of these are consolidated into one piece. It 

 often happens, however, that other deposits 

 are formed, especially in the upper part ; and 

 frequently they refuse to merge into the others, 

 continuing then to be distinct through life as 

 separate small bones having their own serrated 

 margins to articulate with the adjoining struc- 

 tures. 



The lateral pieces (those which comprehend 

 the condyles, and lateral and jugular pro- 

 cesses) commence their formation about the 

 fourth month ; and the anterior piece is the 

 last in the order of development. 



The temporal bone (os temporum; Germ, das 

 Schlafenbein.) One is situated on each side 

 of the sphenoid and lower half of the occipital 

 bone ; they complete the base of the cranium 

 and form the inferior part of the sides of the 

 vault. 



For the purposes of description it is usually 

 divided into three portions; one, strong and 

 compact, in the base and between the middle 

 and posterior fossae, the petrous ; a second, 

 tumid and less dense, behind the ear, the 

 mastoid ; and a third rising from the former 

 two, thin and scaly, situated in the temple, 

 the squamous. 



The petrous portion is an elongated, pyra- 

 midal mass, of which two of the surfaces enter 

 into the formation of the cavity of the cranium, 

 and the third is underneath. It is situated on 

 a line which, if prolonged, would extend from 

 behind the ear to the opposite external angular 

 process of the frontal bone ; but it is limited 

 by the body of the sphenoid. It occupies the 

 space between the posterior border of the ala 

 major of the sphenoid and the basilar process 



of the occipital bone, in the angle of which its 

 free extremity is impacted. In its substance 

 is contained the labyrinth of the ear. 



Of the two surfaces which are in the cra- 

 nium, one is superior, the cerebral; the other is 

 posterior, the cerebellar. 



On the cerebral surface near its middle, is a 

 smooth, convex, and transverse elevation (the 

 processus semicircularis), produced by the su- 

 perior semicircular canal of the labyrinth ; 

 immediately in front of this is a depression on 

 which the Gasserian ganglion lies ; more out- 

 wardly and running lengthwise, is a faint 

 sulcus (the sulcus Vidianus), which terminates 

 at a small opening (the hiatus Fallopii) for the 

 entrance of the Vidian nerve into the aque- 

 ductus Fallopii. 



On the cerebellar surface is seen the foramen 

 auditorium internum, the superior and posterior 

 part of the margin of which is more prominent 

 than the anterior, which, in fact, degenerates 

 into a sulcus. It is the commencement of a 

 canal (the meatus auditorius interims) into 

 which pass the acoustic and facial nerves, and 

 the bottom of which is divided by a ridge into 

 two unequal depressions; the upper one being 

 thefossiila parva, in which is the orifice of the 

 aqueduct of Fallopius for the exit of the facial 

 nerve ; the lower one being the fossula jnagna, 

 in which are several minute perforations for 

 the acoustic nerve. Behind the foramen audi- 

 torium is an indistinct slit, which is the ter- 

 mination of the aqueductus vestibuli; above 

 and rather anterior to this slit is a triangular 

 orifice for the entrance of vessels; and below it, 

 extending to the foramen lacerum posticura, 

 is a slight groove. 



Between the cerebral and cerebellar surfaces 

 there is a sharp ridge on which there is a 

 groove (the sulcus petrosus), more evident pos- 

 teriorly than anteriorly ; to the ridge is attached 

 the tentorium ; the groove lodges the petrosal 

 sinus. 



The under surface is divided into two parts 

 by a sharp, prominent ridge, which has on 

 either side of it a considerable fossa. That on 

 its outer side is the fossa parotidea for the 

 upper part of the parotid gland ; that on its 

 inner side is a thimble-like depression (the 

 fossa jugularis), which forms with the occipital 

 bone the foramen lacerum posterius. In this 

 bone, however, it is not so wide as it is in the 

 occipital ; from which it results that the fora- 

 men is imperfectly divided into two parts - 

 the anterior for the nerves, the posterior fop 

 the vein ; and it is the latter organ which is 

 lodged in the fossa jugularis of the temporal 

 bone The fossa parotidea is limited, above 

 and in front, by a fissure (ihejissura Glasseri), 

 which penetrates to the tympanum and gives 

 exit to the chorda tympani and entrance to 

 the laxator tympani muscle; behind, by the 

 external auditory process. The margin of the 

 foramen auditorium cxternum, which is ellip- 

 tical, has its long diameter vertical, and is 

 the commencement of the meatus auditorius 

 externus ; a tube which is curved a little 

 downwards, is 

 mities than in 



more expanded 

 its middle, and 



at its 

 terminntt.! at 



