PRACTICAL CONSIDERATIONS : THE SKULL. 237 



In trephining- these general facts should be remembered, as should the occa- 

 sional want of parallelism between the inner and outer tables. 



The shape of the skull is influenced by race and by disease. The racial pecu- 

 liarities have sometimes a medico-legal significance, but cannot be described here. 

 (See also page 229.) Pathological asymmetry is caused in many ways. 



In rickets the head is enlarged, and this enlargement seems greater than it 

 really is on account of the retarded growth of the facial bones. All the fontanelles 

 are larger than usual and close later. The anterior fontanelle is sometimes patent 

 at the end of the third or fourth year. 



In craniotabes the rhachitic softening of the bones favors absorption under 

 pressure. Consequently the regions most affected by the thinning of the bones are 

 the occipital and the posterior half of the parietal, which are between two forces, — 

 the expanding and growing brain within and the supporting surface, as the pillow, 

 without. Various peculiar shapes may result. 



The changes in hydrocephalic and microcephalic skulls have already been 

 described. 



Syphilis in the young affects especially the fronto-parietal region, producing 

 thickening or nodes of those bones in the vicinity of the anterior fontanelle. This 

 site is probably determined by the vascularity accompanying growth, as this is the 

 last portion of the cranium to become bony. Such nodes are, therefore, analogous 

 to the rings or collars that form in the long bones of syphilitic children near the 

 epiphyses ; the immobility of the cranial bones, however, causes the exudate to 

 harden rather than to take on inflammatory action. The bulging of the forehead in 

 5ome hereditary syphilitics is due to the catarrh of the frontal sinuses which often 

 accompanies the Schneiderian catarrh, that produces first the so-called "snuffles" 

 and later caries of the nasal bones, with the characteristic flattening of the nose. 



In adults syphilis of the cranium usually causes necrosis, spreading from the 

 external to the internal table. Necrosis from whatever cause is more apt to affect 

 the external table, which is more exposed to injury and less richly supplied with 

 blood. 



The calvaria is far more frequently attacked by disease than the base, doubt- 

 less from its greater liability to traumatism. 



The bones of the cranium are supplied with blood by arteries entering from the 

 pericranium on one side and from the dura mater on the other. The dural supply 

 is the larger ; hence the foramina on the inside of the cranium are larger and more 

 numerous than those on the exterior, and hence also traumatic detachment of the 

 pericranium over considerable areas may not result in necrosis. When detached 

 from disease, the latter (as in syphilis), even when originating externally, is apt to 

 spread along the vessels, and thus cause necrosis by finally affecting the dural supply. 



The meningeal blood-vessels running on the exterior surface of the dura — the 

 remnant of the primitive membranous cranium (Humphry) — and sending branches 

 to the cranium are nottvery strong, and consequently do not offer much resistance 

 to the separation of the dura from the skull ; neither do their branches furnish a 

 very large quantity of blood, surgically considered. It follows that a traumatic 

 separation of the dura is not in itself a lesion followed by serious consequences 

 unless the separation takes place at or about the situation of the main trunks. 

 Hence, when an extradural clot is suspected to be the cause of grave symptoms, it 

 is usually sought for first over the anterior inferior angle of the parietal bone, — i.e., 

 about three centimetres (approximately one inch and a quarter) behind the external 

 angular process on a level with the upper border of the orbit. This will make 

 accessible the region of the main trunk and the anterior branch of the middle 

 meningeal. This latter branch at this point runs through a bony canal on the inner 

 surface of the cranium, and is therefore frequently torn when fracture occurs in this 

 region. An opening on the same level, but just below the parietal eminence, will 

 permit the posterior branch to be reached. 



The venous channels (emissary veins) connecting the sinuses within and the 

 superficial veins without the cranium sometimes convey infective disease, such as 

 erysipelas or cellulitis of the scalp, and thus bring about a septic meningitis or sinus 

 thrombosis. 



