86 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



scapula 



clavicle 

 procoracoid 



coracoid 



glenoid fossa 



humerus 



radius 



sternum. The sternum occurs only in lung-breathing vertebrates, that is to say, it is absent 

 in fishes. There is some doubt as to the origin of the sternum. According to the prevailing 

 view, given in K and W, the sternum of the higher vertebrates is produced by the fusion of the 

 ventral ends of the ribs, i.e., of the costal cartilages. But since in the lower vertebrates the ribs 

 are short and do not reach to the ventral side, this theory necessitates the invention of a different 

 theory for the origin of the sternum of Amphibia. The sternum of Amphibia is consequently 

 supposed to have arisen through the fusion of certain cartilages present in the ventral 

 myosepta of urodeles. See W, pages 140-43, for details of this idea. Recently, however, 

 evidence has been brought forward in favor of the conception that the sternum has the same 

 origin in all of the vertebrates and that its origin is independent of the ribs. According to this 



theory the sternum is the median ventral portion of 

 the pectoral girdle; it has separated from the girdle 

 and extended posteriorly; it may or may not con- 

 nect secondarily with the ribs 1 (see Fig. 26). 



2. The parts of a typical girdle and fore 

 limb. The pectoral girdle is somewhat more com- 

 plicated in structure than the pelvic girdle. It 

 arises in part from the ossification of a cartilagi- 

 nous bar or plate. In this plate as in the case of 

 the pelvic plate three pairs of bones two ventral 

 pairs and one dorsal pair arise. These bones are 

 on each side: a dorsal bone, the scapula, an anterior 

 ventral bone, the procoracoid or precoracoid, and a 

 posterior ventral bone, the coracoid (see Fig. 27). 

 The scapula is analogous to the ilium, the procora- 

 coid to the pubis, and the coracoid to the ischium. 

 These bones like the pelvic bones are cartilage 

 bones since they arise in cartilage. In the case of 

 the pectoral girdle, unlike the pelvic, only two of the 

 three cartilage bones persist in the vertebrate series. 

 Of these the scapula is always present. Of the two 

 ventral pairs of elements only one pair is present in 

 the majority of living groups of reptiles, in birds, 

 and in mammals. It is commonly supposed that 

 the element which persists is the coracoid and that 

 the procoracoid is missing. If this theory be cor- 

 rect, there are hardly any living vertebrates with 



a separate ossified procoracoid. However, there is considerable evidence that the per- 

 sistent element is really the procoracoid in present reptiles, and that in this group the coracoid 

 is missing. In mammals, however, the matter is disputed, some regarding the persistent 

 element as the procoracoid, others as the coracoid (Fig. 28). This persistent ventral cartilage 

 bone of the pectoral girdle of reptiles, birds, and mammals is called the coracoid, and this name 

 will be retained here, with the understanding that in reptiles it is almost certainly homologous 

 with the procoracoid. In nearly all mammals the coracoid is rudimentary, the only cartilage 

 bone of the girdle which retains its full importance being the scapula or shoulder blade. Dia- 

 grams illustrating the hoinology and parts of the girdles of various forms are given in 

 Figure 29. 



In addition to the cartilage bones present in the girdle some of the original cartilage is 

 likely to persist unossified. The cartilages of most common occurrence are: the suprascapula, 



1 F B. Hanson, American Journal of Anatomy, XXVI (1919), 41. 



metacarpals 



phalanges 



FIG. 27. Diagram of the bones of the 

 typical pectoral girdle and fore limb. The 

 cartilage bones of the girdle are stippled, 

 the membrane bone (clavicle) left blank. 

 All of the limb bones are cartilage bones. 

 (From Parker and Haswell's Textbook of 

 Zoology, courtesy of the Macmillan Com- 

 pany.) 



