496 THE PECTORAL GIRDLE. 



Gotte, which tend to shew the keel of tlie sternum is really an interclavicle, 

 appear to me of great importance. 



A prsecoracoid, partially separated from the coracoid by a s-pace, is 

 present in Struthio. It is formed by a fenestration of a primitively 

 continuous cartilaginous coracoid plate (Hoffmann). In Dromseus and 

 Casuarius clavicles are jn-esent (fused with the scapula in the adult 

 Dromseus), though absent in other Ratitaj (Parker, etc.). 



Mammalia. The coracoid element of the coraco-scapular plate is 

 much reduced in Mammalia, forming at most a simple process (except in the 

 Ornithodelphia) which ossifies however separately'. 



With refer-ence to the clavicles the same divergencies of opinion met 

 with in other types are found here also. 



The clavicle is stated by Rathke to be at first continuous with the 

 coraco-scapular plate. It is however soon sepai'ated, and ossifies very early, 

 in the human embryo before any other bone. Gegeubaur however 

 shewed that the human clavicle is provided with a central axis of 

 cartilage, and this observation has been confirmed by Kolliker, and ex- 

 tended to other Mammalia by Gotte. The mode of ossification is never- 

 theless in many respects intermediate between that of a true cartilage bone 

 and a membrane bone. The ends of the clavicles remain for some time, 

 or even permanently, cartilaginous, and have been interpreted by Parker, 

 it appears to me on hardly sufficient grounds, as parts of the mesoscapula 

 and priecoracoid. Parker's so-called mesoscapula may ossify sej)arately. 

 The homologies of the episternum are much disputed. Gotte, who has 

 worked out the development of the parts more fully than any other 

 anatomist, finds that paired interclavicular elements grow out backwards 

 from the ventral ends of the clavicles, and uniting together form a some- 

 what T-shaped interclavicle overlying the front end of the stern vim. This 

 condition is pex'manent in the Ornithodelphia, except that the anterior part 

 of the sternum undergoes atrophy. But in the higher forms the inter- 

 clavicle becomes almost at once divided into three parts, of which the two 

 lateral remain distinct, while the median element fuses with the subjacent 

 part of the sternum and constitutes with it the presternum (manubrium 

 sterni). If Giitte's facts are to be trusted, and they have been to a large 

 extent confirmed by Hoffmann, his homologies ap})ear to be satisfactorily 

 established. As mentioned on p. 463 Ruge (No. 438) holds that Gcitte is 

 mistaken as to the origin of the presternum. 



Gegenbaur admits the lateral elements as parts of the interclavicle, 

 while Parker holds that they are not j)arts of an interclavicle but are 

 homologous with the omosternum of the Frog, which is however held by 

 Gotte to be a true interclavicle. 



Amphibia. In Amphibia the two halves of the shoulder girdle are 

 each formed as a continuous plate, the ventral or coracoid part of which is 

 forked, and is composed of a larger posterior and a smaller anterior bar-like 

 process, united dorsally. In the TJrodela the two remain permanently free 

 at their ventral ends, but in the Anura they become united, and the space 

 between them then forms a fenestra. The anterior process is usually 



^ This process, known as the coracoid process, is held by Sabatier to be the 

 prascoracoid ; while this author also holds that the upper third of the glenoid cavity, 

 which ossifies by a special nucleus, is the true coracoid. Tlie absence of a pra?coracoid 

 in the Ornithodelphia is to my miud a serious difficulty in the way of Sabatier's view. 



