60 



COMPARATIVE ANATOMY 



ing to differentiations of the pectoral arch/ but as consisting of 

 skeletal parts which primarily belong to the body-wall, and only 

 secondarily come into connection with the limb-skeleton. 



In most Urodeles and certain Anurans the edges of the cartilag- 

 inous sternum are inserted into the grooved median margins of the 

 two coracoids (Fig. 43, B, C), to which they are united by connective 

 tissue. In Rana, on the other hand (D), in which the two halves 

 of the pectoral arch are much moi'e closely connected in the 

 middle line, by far the greater part of the sternum lies entirely 

 posterior to the coracoids. In the Perennibranchiata and Dei'O- 

 tremata the sternum is much simpler than in other Amphibians, 

 and in Proteus and Amphiuma it undergoes complete degeneration. 



Fig. 44.- 



-Peotosal Arch and Stebnum op a Gecko 

 ( Hemidactylux verrucosus). 



St, sternum : R, ribs ; Si, cartilaginous cornua to which the last pair of ribs is 

 attached ; SS, suprascapula ; <?, scapula ; Co, coracoid ; Go'^, cartilaginous 

 epicoracoid : Ep, episternum ; a, h, c, membranous fenestrfe in the coracoid ; 

 01, clavicle ; G, glenoid cavity for the humerus. 



Nothing is known with regard to the sternum of fossil Amphibians, 

 which was probably entirely cartilaginous. 



In the Amniota, the sternum arises by a number of ribs on 

 either side of tlie middle line running together to form a continuous 

 cartilaginous tract. An unpaired cartilaginous sternal plate is 

 formed by the tract of either side becoming more or less completel}' 

 fused with its fellow, and from this plate the ribs become 

 secondarily segmented off by the formation of true articulations. 



1 It has been recently shown that in the Elasmobranch Xotidaini.i cartilages 

 are present in the median ventral line of the pectoral arx)h which are segmented 

 off from the coracoids. 



