104 VERTEBEATA. 



united to one another in front, and articulate separately 

 with the skull behind. In many cases, each half, or "ramus," 

 of the lower jaw consists of several pieces united to one 

 another by sutures ; but in the Mamnudia each ramus con- 

 sists of no more than a single piece. The two rami are very 

 variously connected with one another, being sometimes only 

 joined by ligaments and muscles, sometimes united by car- 

 tilage or by bony suture, and sometimes fused or anchylosed 

 with one another, so as to leave no evidence of their true 

 composition. The mode by which each ramus of the lower 

 jaw articulates with the skull also varies. In the Mammalia 

 the lower jaw articulates with a cavity formed on what is 

 known to human anatomists as the temporal bone ; but in 

 Birds and Reptiles the lower jaw articulates with the skull, 

 not directly, but by the intervention of a special bone, known 

 as the " quadrate bone " or " os quadratum." 



As regards the limhs of Vertebrates, whilst many differ- 

 ences exist, which will be afterwards noticed, there is a 

 general agreement in the parts of which they are composed. 

 As a rule, each pair of limbs is joined to the trunk by means 

 of a series of bones which also correspond to one another in 

 general structure. The fore-limbs, often called the "pectoral" 

 limbs, are united with the trunk by means of a bony arch, 

 which is called the " pectoral " or " scapular " arch ; whilst 

 the hind-limbs are similarly connected with the trunk by 

 means of the " pelvic arch." In giving a general description 

 of the parts which compose the limbs and their supporting 

 arches, it will be best to take the case of a Mammal, and the 

 departures from this type will then be readily recognised. 



The pectoral or scapular arch consists usually of three 

 bones, the " scapula " or shoulder-blade, the " coracoid," and 

 the " clavicle " or collar-bone ; but in the great majority of 

 the Mammals, the coracoid is anchylosed with the scapula, of 

 which it forms a mere process. The scapula or shoulder-blade 

 (fig. 482, s) is usually placed outside the ribs, and it forms, 

 either alone or in conjunction with the coracoidal element of 

 the shoulder-girdle, the cavity with which the upper arm is 

 articulated. The coracoid, though rarely existing as a distinct 

 bone in the Mammals, plays a very important part in other 



