i8o THE SKULL. [chap. 



an extraordinary development of air-cells takes place in the 

 cancellous tissue between the outer and inner surface walls or 

 " lables " of nearly all the bones of the cranium, separating 

 them in some cases as much as twelve inches apart (see Fig. 

 60). These cells are not only formed in the walls of the 

 cranium proper, but are also largely developed in the nasal 

 bone and upper part of the premaxilla and maxilla, the 

 bones forming the palate and the basicranial axis, and even 

 extend into the interior of the ossified mesethmoid and the 

 vomer. Where two originally distinct bones come in contact 

 the cells pass freely from one to the other, and almost all the 

 sutures become completely obliterated in old animals. 



The intercellular lamellae in the great mass which sur- 

 rounds the brain-cavity superiorly and laterally mostly 

 radiate from the inner to the outer table, but in the other 

 bones their direction is more irregular. Like the similar 

 but less developed air-cells in the skulls of many other 

 Mammals, they are entirely secondary to the original growth 

 of the bones. In the young African Elephant's skull figured 

 (from an animal about six months old, see Fig. 59), their 

 formation has scarcely commenced, and as the sutures are 

 still quite distinct, and the bones not distorted by these 

 cellular dilatations, they are in a much better state for 

 studying their connections and characteristics. 



When the basicranial axis is placed in a horizontal 

 position, it will be seen that the foramen magnum is quite 

 posterior, and its plane nearly vertical. The cranial cavity 

 is elongated and depressed (more so in the African than the 

 Indian Elephant), the tentorial plane nearly vertical, so that 

 the cerebellar fossa is altogether behind the cerebral fossa. 

 The latter is broad behind and contracted laterally in front. 

 The olfactory fossa is large, and placed altogether below 

 the anterior part of the cerebral fossa, the cribriform plate 



