GENERAL CHARACTERS OF VERTEBRATA. 303 



cles, sometimes united by cartilage 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 Eeptiles, the lower jaw articu- 

 lates with the skull, not directly, but by the intervention of a 

 special bone, known as the "quadrate bone" or "0s quadratum." 



As regards the limbs of Vertebrates, whilst many differences 

 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. 270, s) is usually placed outside the ribs, and it 

 forms, either alone or in conjunction with the other bones 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 

 Vertebrates. The clavicles are often wanting, or rudimentary, 

 and they are the least essential elements of the scapular arch. 

 The fore-limb proper consists, firstly, of a single bone which 

 forms the upper arm, and which is known as the humerus (Ji). 

 This articulates above with the shoulder-girdle, and is followed 

 below by the fore-arm, which consists of two bones, called the 

 radius and ulna. Of these the radius is chiefly concerned 

 with carrying the hand. The radius and ulna are followed by 

 the bones of the wrist, which are usually composed of several 

 bones, and constitute what is called the carpus (d). These 

 support the bones of the root of the hand, which vary in 

 number, but are always more or less cylindrical in shape. 

 They constitute what is called the metacarpus. The bones of 



