CRANIOLOGY 



199 



The degree of projection of the jaws is determined by the gnathic or alveolar index, which repre- 

 sents the proportion between the basialveolar and basinasal lengths, thus: 



basialveolar length X 100 



basinasal length 



The following table, modified from that given by Duckworth, 1 illustrates how these different 

 indices may be utilized in the classification of skulls: 



The chief function of the skull is to protect the brain, and therefore those portions of the skull 

 which are most exposed to external violence are thicker than those which are shielded from injury 

 by overlying muscles. Thus, the skull-cap is thick and dense, whereas the temporal squamae, 

 being protected by the temporales muscles, and the inferior occipital fossae, being shielded by the 

 muscles at the back of the neck, are thin and fragile. Fracture of the skull is further prevented 

 by its elasticity, its rounded shape, and its construction of a number of secondary elastic arches, 

 ^ach made up of a single bone. The manner in which vibrations are transmitted through the 

 bones of the skull is also of importance as regards its protective mechanism, at all events as far 

 us the base is concerned. In the vault, the bones being of a fairly equal thickness and density, 

 vibrations are transmitted in a uniform manner in all directions, but in the base, owing to the 

 ^arying thickness and density of the bones, this is not so; and therefore in this situation there 

 :ire special buttresses which serve to carry the vibrations in certain definite directions. At the 

 iront of the skull, on either side, is the ridge which separates the anterior from the middle fossa 

 of the base; and behind, the ridge or buttress which separates the middle from the posterior fossa; 

 and if any violence is applied to the vault, the vibrations would be carried along these buttresses 

 10 the sella turcica, where they meet. This part has been termed the "center of resistance," 

 and here there is a special protective mechanism to guard the brain. The subarachnoid cavity 

 at the base of the brain is dilated, and the cerebrospinal fluid which fills it acts as a water cushion 

 to shield the brain from injury. In like manner, when violence is applied to the base of the skull, 

 as in falls upon the feet, the vibrations are carried backward through the occipital crest, and 

 forward through the basilar part of the occipital and body of the sphenoid to the vault of the skull. 



In connection with the bones of the face a common malformation is cleft palate. The cleft 

 usually starts posteriorly, and its most elementary form is a bifid uvula; or the cleft may extend 

 through the soft palate; or the posterior part of the whole 

 of the hard palate may be involved, the cleft extending as 

 ;'ar forward as the incisive foramen. In the severest forms, 

 :he cleft extends through the alveolus and passes between 

 :he incisive or premaxillary bone and the rest of the max- 

 illa; that is to say, between the lateral incisor and canine 

 ^eeth. In some instances, the cleft runs between the central 

 and lateral incisor teeth; and this has induced some 

 anatomists to believe that the premaxillary bone is devel- 

 oped from two centers (Fig. 199) and not from one, as was 

 stated on p. 163. The medial segment, bearing a central 

 incisor, is called an endognathion; the lateral segment, bear- 

 ing the lateral incisor, is called a mesognathion. The cleft 

 may affect one or both sides; if the latter, the central part is frequently displaced forward and re- 

 mains united to the septum of the nose, the deficiency in the alveolus being complicated with a cleft 



1 Morphology and Anthropology, by W. L. H. Duckworth, M.A., Cambridge University Press. 



Endognatliion 

 Mesognathion 

 ExognatM< 



FIG. 199. The premaxilla and its sutures. 

 (After Albrecht.) 



