IX.] OF THE DOG. 127 



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. 60, 

 p. 181), where the alteration of the external form of skull 

 during growth, without material change in the shape or size 

 of the cerebral cavity, is strikingly shown. At the same 

 time the alveolar borders of the jaws gradually enlarge to 

 adapt themselves to the increased size of the permanent 

 teeth which they have to support, and the various ridges and 

 tuberosities for the attachment of muscles become more 

 prominent. 



During these changes a gradual consolidation takes place 

 in the structure of the skull generally, by the partial or com 

 plete union of certain of the bones by synostosis. The 

 union of the different bones generally proceeds in a certain 

 definite order, which, however, varies much in different 

 species. Sometimes it extends so far as to lead to complete 

 obliteration of all the cranial sutures. 



