. Elements of Comparative 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
. Elements of Comparative 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
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