442 



COMPAEATIVE ANATOMY. 



Fig. 232. Sternum and Slioulder- 

 girdle of Rana temporaria. p Body 

 of the Sternum, sc Scapula, sc' Supra- 

 scapular. CO Coracoid fused iu the 

 middle line with its fellow of the 

 opposite side (s). cl. Clavicle, e Epi- 

 sternum. The cartilaginous parts are 

 shaded. 



Sternum. 



§ 338. 



The Sternum forms the ventral portion of the framework of 

 archeSj which is formed by the ribs. It is developed from a rudi- 

 ment similar to that of the ribs, 

 as a band of cartilage, which con- 

 nects together the proper ribs on 

 each side. It appears, therefore, 

 as a paired portion of the skeleton, 

 and its later characters are due 

 to its fusion along the middle 

 line. We first meet with it in 

 the Amphibia. We must there- 

 fore suppose that in these forms 

 there was once a stage, in which 

 the ribs were united by a sternum. 

 Of this stage nothing has been re- 

 tained, in addition to the rudiments 

 of the ribs, except the part which 

 represents the rudiment of the sternum ; and the preservation of 

 this rudiment is explained by the fact that it is connected with the 

 shoulder -girdle. Thus in the Salamandrina it has the form of a 



broad thin plate of cartilage, in which 

 there are deep grooves for the attach- 

 ment of the coracoids. In the Anura 

 (Fig. 232, p) it is placed at the hinder 

 edge of the coracoids (co) which are 

 united together in the middle line, and 

 forms a partly ossified appendage to the 

 shoulder-girdle, the hinder end of which 

 persists as a broad plate of cartilage. 



The sternal plate of the Saurii and 

 Crocodilini resembles the broader form 

 of Amphibian sternum. It is generally 

 rhomboidal in form, and has the same 

 relations to the shoulder-girdle (Fig. 

 233, s). As a rule but few pairs of ribs 

 are connected with the sternum, which 

 frequently retains its cartilaginous con- 

 dition (Fig. 233, s) ; at its hinder edge 

 it gives off one or two processes, Avhich 

 also receive ribs. The paired condition 

 of this second portion of the sternum 

 must be regarded as a continuation of the embryonic arrangement. 



The sternum of Birds is always ossified ; it represents the more 

 highly developed sternal plate of Reptiles, but the hinder portion is 

 no longer developed. It has, moreover, but few pairs (no more than 



Fig. 233. sternum and Shoul- 

 der-girdle of Uromastix 

 spinipes. s Sternal plate, 

 which supports at its sides 

 several pairs of ribs, and is 

 provided posteriorly with two 

 processes, sc Scapula, co Co- 

 racoid. cl Clavicle, t Epi- 

 sternum. The cartilaginous 

 portions of the sternum and 

 coracoids are dotted. 



