148 THE SKULL. [CHAP. 



The "cleft ' between the palato-pterygoid (II.) and the 

 mandibular (III.) bar is the mouth. The first true cleft, 

 however, is the one between the mandibular and the anterior 

 hyoid arch (IV.). It becomes contracted into the Eustachian 

 tube, tympanic cavity, and meatus auditorius externus, which 

 would form a canal of communication between the pharynx 

 and the external surface but for the interposition of the 

 delicate membrana tympani. 



Some of the changes which take place in the cranium 

 while advancing from youth to maturity have already been 

 noticed ; but it will be well, before proceeding to describe 

 the modifications of the Mammalian skull, to mention certain 

 others which take place, to a greater or less degree, in all 

 skulls. 



These depend mainly on the fact that the brain, and con- 

 sequently the cavity which contains it, and also the sense 

 capsules, increase in size in a much smaller ratio than the 

 external parts of the head, especially the jaws and pro- 

 minences for the attachment of muscles. The dispropor- 

 tionate growth and alteration of form of these parts, 

 concomitant with little or no change in the brain-case, is 

 effected partly by increase in thickness of the bones, but 

 mainly by the expansion of their walls and the develop- 

 ment of cells within, which greatly extend the outer surface 

 without adding to the weight of the bone. 



In the Dog these cells are developed chiefly in the fore- 

 part of the frontal bones, constituting the frontal sinuses, 

 and in the presphenoid, constituting the sphenoidal sinuses. 

 Air passes freely into them from the nasal passages. In 

 many animals they attain a much larger extent than in the 

 Dog, reaching their maximum in the Elephant (see Fig. 64, 

 p. 205), where the alteration of the external form of skull 

 during growth, without material change in the shape or size 



