604 THE PECTORAL GIRDLE. 



connecting the furculum with the sternum. The observations of Gotte, 

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

 to me of great importance. 



A praecoracoid, partially separated from the coracoid by a space, is pre- 

 sent in Struthio. It is formed by a fenestration of a primitively continuous 

 cartilaginous coracoid plate (Hoffmann). In Dromaeus and Casuarius cla- 

 vicles are present (fused with the scapula in the adult Dromasus), though 

 absent in other Ratitae (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 1 . 



With reference 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 separated, and ossifies very early, in the 

 human embryo before any other bone. Gegenbaur however shewed that 

 the human clavicle is provided with a central axis of cartilage, and this obser- 

 vation has been confirmed by Kolliker, and extended to other Mammalia by 

 Gotte. The mode of ossification is nevertheless in many respects inter- 

 mediate between that of a true cartilage bone and a membrane bone. The 

 ends of the clavicles remain for some time, or even permanently, cartila- 

 ginous, and have been interpreted by Parker, it appears to me on hardly 

 sufficient grounds, as parts of the mesoscapula and prascoracoid. Parker's 

 so-called mesoscapula may ossify separately. The homologies of the epister- 

 num 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 somewhat T-shaped interclavicle overlying the front 

 end of the sternum. This condition is permanent in the Ornithodelphia, 

 except that the anterior part of the sternum undergoes atrophy. But in the 

 higher forms the interclavicle 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 pre- 

 sternum (manubrium sterni). If Gotte's facts are to be trusted, and they 

 have been to a large extent confirmed by Hoffmann, his homologies appear to 

 be satisfactorily established. As mentioned on p. 563 Ruge (No. 438) holds 

 that Gotte 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 parts of an interclavicle but are homologous 

 with the omosternum of the Frog, which is however held by Gotte to be a 

 true interclavicle. 



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

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

 which ossifies by a special nucleus, is the true coracoid. The absence of a prae- 

 coracoid in the Ornithodelphia is to my mind a serious difficulty in the way of 

 Sabatier's view. 



