72 



COMPARATIVE ANATOMY 



halves of the pectoral arch are much more closely connected in the 

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

 posterior to the coracoids, which do not overlap one another. In 

 the Perennibranchiata and Derotremata the sternum is much 

 simpler than in other Amphibians, and in Proteus and Amphiuma 

 it is entirely wanting. 



In the Amniota, the form of the sternum, like that of the 

 pectoral arch, depends largely on the nature and function of the 

 forelimbs. It is usually considered as arising primarily by a 

 number of ribs running together ventrally so as to form a con- 

 tinuous cartilaginous longitudinal tract on either side. By the 

 more or less complete fusion of these two tracts, an unpaired sternal 



FIG. 56. PECTORAL ARCH AND STERNUM OF A GECKO (Hemidactyhis 



n rriicoaus). From the ventral side. 



o, b, c, membranous fenestne in the coracoid ; Co. coracoid ; Co 1 , cartilaginous 

 epicoracoid ; Cl, clavicle ; Ep, episternum ; G, glenoid cavity for the 

 humerus ; R, ribs ; 8, scapula ; Si, cartilaginous cornua to which the last 

 pair of ribs is attached ; SS, suprascapula ; St, sternum. 



band or plate is formed, from which the ribs are secondarily 

 segmented off by the formation of articulations, and beneath which 

 a dermal episternum is present in some cases (p. 44). The main 

 part of the anterior end of the sternum of Mammals is formed by 

 the median union of the first two or three pairs of cervical ribs, 

 and beneath and in front of this region in Monotremes is a large 

 T-shaped bone, the prosternum (" episternum "), the lateral parts of 

 which come into relation with the clavicles (Fig. 103). l 



The sternum may become calcified (Reptiles), or converted 

 into true bone (Birds, Mammals). In Reptiles,' 2 Birds, and Mono- 



1 Cf. note on p. 44. 



- In (Snakes ami Chelonians there is no trace of a sternum. 



