CRANIUM. 



743 



rence would be to strengthen the part in which 

 (from the situation of the organ) these vibra- 

 tions might, in general, be expected to concur ; 

 and this is the contrivance adopted in the cra- 

 nium, for in the centre of its base there is a qua- 

 drilateral portion (the body of the sphenoid 

 bone) of characteristic massivenessand strength. 



It does not however augment uniformly in 

 its substance from above downwards. The 

 matter is accumulated in dense lines or ribs, 

 which pass to a common centre, and constitute 

 thereby a peculiar skeleton or frame-work of 

 surpassing strength, which admits of the intro- 

 duction of a lighter and more fragile structure 

 in the intervening spaces, and resists the shocks 

 that arrive through the spine, from behind or 

 from above. 



This frame-work is situated almost entirely 

 in the base; the only part which is in the 

 calvarium being a longitudinal curved line, 

 formed by the ethmoidal process of the sphe- 

 noid bone, the crista galli of the ethmoid, the 

 spine of the frontal, the thickened commutual 

 margins of the parietals, and the superior limb 

 of the internal occipital spine. Independently 

 of this curved rib, the calvarium consists of 

 four ovoidal domes, two on each side; formed, 

 the anterior by the corresponding half of the 

 frontal bone, and the posterior by the parietal. 

 The summits of these domes are their centres 

 of ossification, and their bases abut, partly on 

 the longitudinal rib, and partly on the frame- 

 work in the base. 



The part to which all the forces tend is the 

 body of the sphenoid bone. From its posterior 

 corners there pass backwards two ribs, (the 

 petrous processes of the temporal bones,) 

 which terminate on the extremities of an arch, 

 (the lateral limbs of the internal crucial spine 

 of the occiput,) which is placed horizontally, 

 and the convexity of which is turned back- 

 wards. 



This arch and the two ribs which connect it 

 to the centre are in the line in which the oc- 

 ciput would strike the ground in falling back- 

 wards ; and they further form the brim of the 

 pit which contains the cerebellum, so that the 

 vibrations of force pass in the interstice between 

 that organ and the cerebrum. 



From each side of the body of the sphenoid 

 bone there stretches forwards, outwards, and 

 upwards towards the temples, a curved rib, 

 (the anterior part of the great wing,) and, from 

 the anterior part of the body, a transverse rib 

 which overlays the former. These and the 

 posterior lateral ribs, all of which depart from 

 a common centre, constitute the frame-work of 

 the base which sustains the ovoidal domes of 

 the calvaria. The frontal dome is placed with 

 its summit (the frontal depression) looking 

 backwards, downwards, and inwards ; its mar- 

 gin is received, inferiorly on the whole length 

 of the anterior transverse, and on the extremity 

 of the anterior lateral curved rib; towards the 

 middle line, on so much of the longitudinal 

 rib as extends to the parietal bones ; and supe- 

 riorly, it is applied against a portion of the 

 base of the parietal dome. It is against these 

 parts that it thrusts, whenever it receives a 



shock on its summit. The parietal dome is 

 placed with its summit (the parietal depression) 

 looking downwards and inwards. Below, it 

 is received on the extremities of the lateral 

 ribs ; above, it thrusts against the remainder of 

 the longitudinal rib ; behind, it falls on the 

 corresponding portion of the horizontal arch ; 

 and, in front, it antagonizes the frontal. 



It is by the bases of these domes thus 

 thrusting against a solid frame-work, that the 

 cranium is endowed with the power of re- 

 sisting lateral shocks whether they approach 

 from before or behind ; and it is not, as some 

 allege, simply by the mobility of the head, 

 that it withstands blows, which, if it were 

 fixed, would fracture it. 



There yet remains to be noticed an impor- 

 tant part of this skeleton or frame-work ; that 

 which bears upon the spine, and resists the 

 force transmitted through it. At the bottom of 

 the pit containing the cerebellum, there is an 

 elliptical opening (the foramen magnum), the 

 margin of which is very dense; this opening is 

 provided underneath with two tubercles (the 

 articulating processes), by which it rests on the 

 vertebral column ; from these tubercles a curved 

 rib on each side (the lateral process of the oc- 

 cipital bone and the mastoid of the temporal) 

 extends upwards and outwards to the extremity 

 of the posterior lateral rib ; the segment of the 

 margin of the opening which is anterior to the 

 tubercles, is prolonged upwards and forwards, 

 in the form of a broad pillar (the basilar pro- 

 cess), to the back part of the common centre; 

 the segment which is behind the tubercles 

 sends off, at its back part, a spine (the inferior 

 limb of the internal crucial spine), which ends 

 at the centre of the horizontal arch, at the point 

 where the superior longitudinal rib terminates ; 

 and this point of confluence of the forces from 

 below, from above, and from behind, is strength- 

 ened by a nodule (the internal occipital protu- 

 berance). The frame-work of the cerebellar 

 cavity is thus connected with that of the general 

 cavity; anteriorly, to the body of the sphenoid 

 bone; posteriorly, to the tubercle of the occi- 

 pital ; and, laterally, to the extremities of the 

 petrous processes of the temporal bones. In 

 both of them it will be seen that they occupy 

 spaces between the grand divisions of the ner- 

 vous matter, which latter is, therefore, removed 

 from the chance of sustaining injury by shocks, 

 much more completely than it could have been 

 had the parietes been submitted to a progres- 

 sive augmentation of substance from above 

 downwards. As it is, the spaces in which the 

 nervous matter reposes are thin and frequently 

 diaphanous; and, were they situated in un- 

 protected parts, would be perforated by the 

 slightest force. 



During a considerable period of life the sub- 

 ject enjoys additional protection from the slight 

 yielding of the bones, and from the cartilage 

 which intervenes especially at the base. Pres- 

 sure applied on the vertex would tend to disjoin 

 the parietal bones from each other, and from 

 the frontal and occipital bones. This the pe- 

 culiar nature of the articulations forbids, and 

 the longitudinal rib chiefly, and the expanded 



