ANATOMY. 



modate itself to the varying 1 figure of the 

 parts through which it passes, in the act 

 of parturition, and to sustain the violent 

 pressure which it experiences in the same 

 act, is sufficiently obvious. In the pro- 

 gress of ossification the edges of the 

 bones meet each other, and become uni- 

 ted by the sutures. The use of these in 

 the adult cranium cannot be satisfactorily 

 assigned, nor do we see any difference 

 that would arise, if the head had been 

 composed of one piece only, without any 

 suture. In old persons the sutures often 

 become more or less generally obliterated. 



The individual bones are very firmly 

 connected by this mode of union. The 

 edges of the different bones overlap each 

 other at different parts, so that they are 

 mechanically locked together, and can- 

 not be driven in by any force ab externo. 



The bones of the cranium are compo- 

 sed of two plates of compact bony sub- 

 stance, called the external, and internal 

 or vitreous tables; and an intervening 

 more or less obvious reticular texture, 

 termed diploe. The proportion of these 

 constituent parts varies very considerably; 

 the diploe is in no case of a very loose or 

 open tt-xture. The thickness of indivi- 

 dual skulls i's subject to great variety; 

 iind there is much difference in the va- 

 rious pai-ts of the same skull. For the 

 internal surface is every where exactly 

 moulded to the form of its contents, in- 

 stead of influencing them, as we might 

 have expected a priori. Hence the con- 

 volutions of the brain, the vessels which 

 ramify on its surface, &c. all leave prints 

 on the inner table. The ordinary thick- 

 ?iess varies from about the fifth of an 

 inch to almost a mere line. 



The common number of the bones of 

 the cranium is, as we have already stated, 

 7 : but this is often increased by small 

 portions formed between the others, and 

 surrounded by distinct sutures. These 

 are called ossa triquetra, or wormiana. 



The form of the cranium is elliptical, 

 and pretty regularly so, particularly on 

 the front, upper and back part, and sides. 

 The smaller circle of the ellipse is in 

 front, and the larger behind. It is tolera- 

 bly smooth, externally, except its basis, 

 and it is almost entire or unperfbrated, 

 except at the same part. In this situation, 

 however, it possesses numerous holes, or, 

 as they are technically named, foramina, 

 which transmit blood-vessels to the brain, 

 and the nine pairs of nerves which arise 

 from that organ. 



The upper and lateral parts of the era- 

 mum constitute a bony vault or arch, for 



protecting the brain : this part is distin- 

 guished by the name of the skull cap. 



Individual bones of the head. The os 

 frontis forms the upper and anterior part 

 of the skull, the eyebrow, and the roof 

 of the orbit. 



The ossa parietalia are called also ossa 

 bregmatis, since the fontanelles or breg- 

 mata are formed between their edges. 

 They compose the whole upper and most 

 of the lateral parts of the skull, and pos- 

 sess an irregularly quadrangular figure. 



The ossaternporum compose the lower 

 part of the sides, and the middle of the 

 basis of the cranium. They are divided 

 into a squamous portion, a mamillary, and 

 a petrous portion. The former of these 

 has a process contributing to the zygoma, 

 or bony arch, at the side of the cranium, 

 under which the temporal muscle passes. 

 The second is also remarkable, by forming 

 a large nipple-like protuberance towards 

 the basis cranii. The third, which pro- 

 jects into the cavity of the skull, con- 

 tains the organ of hearing. 



The os spheno-occipitale has generally 

 been described as two bones. The occi- 

 pital portion forms the posterior portion 

 of the basis cranii, and a part also of ;he 

 back of the bony ca.se. 



Tlu sphenoid portion is situated in the 

 middle of the base of the skull, and ex- 

 tends across it from one temple to another. 

 Jt is extremely irregular in its figure, and 

 divided into a body placed in the middle, 

 two alae on the sides, and two pterygoid 

 processes projecting downwards. 



The os ethmoides ; occupies the middle 

 of the forepart of the basis cranii. It lies 

 in the interval between the two orbits, and 

 contributes to the cavity of the nose. It 

 consists of an irregular assemblage of 

 bony cells and processes, of a very thin 

 and delicate formation. It has a cribri- 

 form or horizontal plate towards the brain : 

 a nasal or perpendicular plate ; two turbi- 

 nated bones; cells; and two orbital plates. 



The stutures joining these are the co- 

 ronal, between the os frontis and the 

 two ossa parietalia; the sagittal, be- 

 tween the two ossa parietalia; the lamb- 

 doidal, joining the ossa parietalia to the 

 os occipitis ; the squamous, between the 

 temporal and parietal bones. 



The foramina occurring in the cranium, 

 for the transmission of nerves, are; 1, 

 those of the cribriform plate of the eth- 

 moid bone : 2, f. optica : 3, f. lacera or 

 bitalia : 4, f. rotunda : 5, f. ovalia : 6, 

 meatus auditorii interni : 7, f. lacera in 

 basi cranii : 8, f. condyloidea anteriora : 

 9, foramen magnum. 



