SKELETON AND TEETH 67 



portions, the hinder one of which terminates in the condyle, 

 a rounded, usually semicylindrical projection, which fits into 

 the glenoid cavity of the squamosal. The anterior portion 

 of the ascending ramus ends above in the coronoid process, which 

 serves for the insertion of the temporal muscle, the upper 

 portion of which is attached to the walls of the cranium and 

 thus, when the muscle is contracted, the jaws are firmly closed ; 

 the coronoid process passes inside of the zygomatic arch. The 

 lower jaw is therefore a lever of the third order, in which the 

 power is applied between the weight {i.e. the food, the resistance 

 of which is to be overcome) and the fulcrum, which is the 

 condyle. At the postero-inferior end of the ascending ramus 

 is the angle, the form of which is characteristically modified 

 in the various mammalian orders and is thus employed for 

 purposes of classification. 



The hyoid arch is a U-shaped series of small and slender 

 bones, with an unpaired element closing the arch below ; 

 each vertical arm of the (J is attached to the tympanic of its 

 own side and the whole forms a flexible support for the tongue, 

 but with no freely movable joint like that between the lower 

 jaw and the squamosal. 



The mammalian skull in its primitive form may be thought 

 of as a tube divided into two parts, of which the hinder one is 

 the brain-chamber, or cranial cavity, and the forward one the 

 nasal chamber or passage. With the growth of the brain and 

 consequent enlargement of the cranium, this tubular character 

 is lost ; and various modifications of the teeth, jaws and facial 

 region, the development of horns and tusks, bring about the 

 many changes which the skull has undergone. 



This brief sketch of the skull-structure is very incomplete, 

 several of its elements having been altogether omitted and only 

 those parts described which are needful in working out the 

 history and descent of the various mammalian groups. 



The second portion of the axial skeleton is the backbone, 

 or vertebral column, which is made up of a number of separate 



