CRANIUM. 



741 



of the sphenoid bone vertically) at once per- 

 ceive the same analogy to exist. If, when 

 they are thus placed, we look at the cerebral 

 surface, we shall recognize the body in that of 

 the sphenoid ; the vertebral plates in the small 

 wings of the sphenoid, and two halves of the 

 frontal bone; the foramen in the space cir- 

 cumscribed by these last ; the transverse 

 processes in the two great wings of the sphe- 

 noid ; and the notches in the lacerated orbitar 

 foramina, and the angles between the body of 

 the sphenoid and posterior margin of its great 

 wings. If we look at it in front, it will not 

 require any great stretch of the imagination 

 to recognize the four articulating processes in 

 the pterygoid processes of the sphenoid bone 

 and the external angular processes of the 

 frontal. 



The temporal and the parietal bones toge- 

 ther represent another vertebra, situated be- 

 tween the former two. By looking at the 

 base of the skull held vertically, and abstract- 

 ing in the mind the occipital bone, we can 

 (under favour of the license allowed to, or 

 taken by anatomists) see in the two petrous 

 portions of the temporal bones, if they were 

 brought into contact, a type of the body of a 

 vertebra; and in those parts of them which 

 contribute to form the anterior and posterior 

 lacerated foramina, we observe a resemblance 

 to those notches which form in the vertebra, 

 as they do here, conjugal foramina. The arti- 

 cular eminences of the temporal bones give 

 us no bad notion of the transverse processes, 

 while the zygomatic processes above (still 

 holding the skull vertically) and the part 

 which projects behind the mastoid processes 

 below, will indicate the four oblique or arti- 

 culating processes. Lastly, the squamous pro- 

 cesses of the temporal and the whole of the 

 parietal bones represent the vertebral plates, 

 and the space enclosed by them, the vertebral 

 foramen. 



Development of the cranial bones. The 

 progressive development of the bones of the 

 cranium has been pointed out in their separate 

 descriptions ; but there are some general facts 

 which regard its formation as an entire organ 

 which merit further notice. 



The cranium of the fetus presents, like all 

 other organs, a rude outline of the shape it is 

 destined to assume; and, at the earliest pe- 

 riod at which it is noticed, its walls are com- 

 pletely membranous, being formed by the dura 

 mater and pericranium so united as to render 

 it impossible to separate them without injury. 

 Very early points of ossification are developed 

 in this membranous envelope, whence osseous 

 radii shoot out, so that the several points 

 enlarge towards each other, and ultimately 

 coalesce or are united by suture. 



Unlike other bones of a similar character 

 the opposite surfaces are not of similar den- 

 sity. The surface secreted by the vessels of 

 the dura mater contains less animal matter 

 than that which is produced from the vessels 

 of the pericranium ; and it is, therefore, of a 

 more dense and brittle character ; so much so, 



that, when the contiguous bones approximate, 

 the edges of the inner table are simply in 

 juxta-position, a slight layer of cartilage alone 

 separating them. Ilence, in the interior of the 

 skull, the sutures are plain lines ; or, if at all 

 irregular, there is no interchange of substance 

 between them. Not so, however, with the 

 external. By reason of the greater quantity 

 of animal matter which it possesses, and the 

 more diffuse character of its texture, a prin- 

 ciple of toughness is conferred on it which 

 admits of its being dove-tailed with the same 

 table of other bones. 



The base takes precedence of the calvaria 

 in the commencement and completion of its 

 ossification. With the exception of its most 

 prominent points, and the ethmoid bone, it is 

 completely ossified at birth ; while, between 

 the bones of the calvaria, there are conside- 

 rable membranous interspaces, so as to allow 

 of these bones being squeezed together, or to 

 overlap each other, at the period of parturition. 

 The ossific matter departing from the pro- 

 tuberances of the frontal and parietal bones 



and radiating to- Fig. 375. 



wards the circum- 

 ference of these 

 bones, it follows 

 that the angles 

 will be incomplete 

 when the rest of 

 the bone is formed. 

 On this account it 

 is that, at the four 

 angles of each pa- 

 rietal bone, there 



is a membranous spot which the ossific matter 

 has not reached, when, in other parts, it is 

 joined to the surrounding bones. These 

 spaces are called fontanelles ; two of them are 

 situated on the median line and superiorly ; 

 and two others inferiorly and in each lateral 

 region. The posterior superior fontanelle is 

 triangular, and is found between the superior 

 angle of the occipital bone, and the occipital 

 angles of the two parietal. The anterior 

 superior fontanelle (a, 

 Jig. 376), by reason of 

 the frontal bone being 

 formed in two parts, is 

 of a lozenge shape ; and 

 it is between those two 

 parts and the frontal an- 

 gles of the parietal bones 

 that it occurs. These two 

 fontanelles are conse- 

 quently at the extremities 

 of the sagittal suture. 

 The inferior fontanelles 

 are found, the anterior (a, fig. 375) between 

 the spinous angle of the parietal, and the great 

 wing of the sphenoid bone ; the posterior 

 (b, fig. 375) between the mastoid angle of the 

 first-named bone and the mastoid process of 

 the temporal. These two fontanelles are, 

 therefore, situated at the extremities of the 

 squamous suture. 



Fig. 376. 



