236 HUMAN ANATOMY. 



and form a distinct type characterized by the proportionate largeness of the facial 

 bones, the contraction of the brain-case, especially in front and above, the upward 

 slant of the occipital bone between the foramen magnum and the occipital crest, the 

 projection backw'ard of the frontal bone between the parietals at the situation of the 

 anterior fontanelle, and by many minor peculiarities. 



In spite of these, however, they are easily referred to the human species by the 

 descent of the cranial cavity below the level of the glenoid fossa, the number of the 

 nasal bones, the shape of the jaws, the number and direction of the teeth, etc. 



Cretinism is said to be associated with initial deformities of the base pertaining 

 to errors of development and trophic changes in the bones arising from cartilage, 

 especially the basilar process of the occipital and the body of the sphenoid. 

 Accessory to these deviations, and in a measure dependent upon them, are the 

 modified facial proportions and dental irregularity of cretins. 



The Wormian bones, "detached centres of ossification in the marginal area of 

 growing membrane bones, which they aid in occupying intervening spaces among 

 the bones themselves," have been depressed in injuries of the skull, and have 

 resembled fragments of bone pressing against the meninges. The edge of such a 

 bone has been mistaken for a line of fracture. The most frequent cause of the 

 formation of Wormian bones is the stretching of the membranous envelope of the 

 cranial cavity which occurs in hydrocephalus, assistance in the completion of the 

 cranial cavity being supplied by Wormian bones, which may form in numbers, espe- 

 cially along the sagittal, lambdoidal, and squamous sutures. 



The fact that in development the cranial bones touch first and unite first at the 

 points nearest their centres of ossification explains the formation and situation of the 

 fontanelles. The four sides of each parietal bone, for example, become united to the 

 four surrounding bones earlier in the middle than at the four angles. At the latter, 

 therefore, there remain spaces covered with membrane. 



The anterior fontanelle, at the junction with the frontal of the antero-superior 

 angles of the parietal, is the largest, and is not closed for from one to two years after 

 birth. In rickets its closure is much retarded. Its condition, as to fulness or the 

 reverse, gives a Valuable indication in many of the diseases of children. In a state 

 of health, the opening, while still membranous, is level with the cranial bones or is 

 very slightly depressed. Systemic exhaustiorJ* malnutrition, diseases associated 

 with depletion of the vascular system, gastric catarrh, chronic diarrhoea, and maras- 

 mus, or simple atrophy, all produce a marked depression of the fontanelle, which in 

 the great majority of cases indicates feeding and stimulation. 



A bruit de souffle of greater or less intensity, and synchronous with the pulse, is 

 often heard over the anterior fontanelle, and w^as at one time thought to be charac- 

 teristic of rickets and of hydrocephalus, but has little diagnostic significance. 



The thickness of the skull varies in individuals, in the various portions of the 

 skull, and often even in the two halves of the same skull. 



Humphry observes that, as he has often found the skull to be thick in idiots, 

 and the several bones to be thickest when the skull is small, — i.e., when the brain is 

 small, — " the term ' thick-headed,' as a synonym for ' stupid,' derives some confirma- 

 tion from anatomy." Anderson says, however, that the weight of the brain does 

 not seem to have any relation to the thickness of the skull, although this does not 

 af?ect the truth of the statement that as the brain diminishes with age the skull is apt 

 to thicken, the addition of bone taking place on the interior and giving rise to the 

 irregular surface with close dural adhesions often met with in operations upon the 

 cranium in old persons. 



The middle cerebral fossa, the centre of the squamous portion of the temporal, 

 and the middle of the inferior occipital fossae are the thinnest parts of the skull, 

 varying from 1.75 millimetres to .85 of a millimetre, and in exceptional skulls meas- 

 uring only .2 millimetre in thickness. This has an important bearing on the location 

 of fractures (page 239). At the parietal eminence, the posterior superior angle of 

 the parietal, the superior angle of the occij^ital, and especially at the frontal eminences 

 and the occipital protuberance areas of thickening are found ; at the latter point the 

 skull may measure fifteen millimetres in thickness (Anderson). The average thick- 

 ness of the remaining parts of the calvaria is from five to 7.5 millimetres. 



