474 COMPARATIVE ANATOMY. 



appendage lias still some relations to tlie head, as is shown by 

 the muscles which ai'e supplied by cerebral nerves, while, in 

 Fishes, it, and its arch, lie just behind the branchial arches. In 

 this respect the hinder appendages are quite independent. They 

 must be supposed to have travelled to a greater distance, if we are 

 right as to the homodynamy, which a comparison of the skeletons 

 leads us to infer. The anterior appendage has, however, clearly 

 also undergone great changes in position; this is evident when 

 we note how, owing to the continual increase in the number of the 

 cervical vertobrte, it moves farther and farther back as we pass from 

 Fishes to Birds. As there are no .facts known to us which 

 point to the formation of new vertebrae, which could only be brought 

 about by the intercalation of new metameres of the body, this 

 distinct change in position must be explained as due to the continual 

 retrogression of the appendages ; in other words, we are led to 

 postulate just the same process in its case, as in that of the hind 

 limbs. These considerations merely point to the manner in which it 

 is possible that the appendages were developed, and there are still 

 many questions which cannot be safely answered until a compara- 

 tive examination of the muscles and nerves which belong to the 

 appendages has been made. 



Gegenbaue, C, Zur Morphol. der GHedmassen der Wirbelthiere. Morphol. 

 Jahrb. II. 



Anterior Appendages. 



Shoulder- Girdle* 

 § 358. 



In its simplest form the shoulder- girdle is a piece of cartilage, 

 which, in the Selachii, forms an arch on each side united to its 

 fellow along the ventral line, and placed just behind the branchial 

 apparatus. Owing to its attachments to the muscles of the appen- 

 dages, definite sculpturing may be made out on the arch; this is 

 most distinct in the Rays. 



In the Ganoidei the two halves of the cartilaginous arch are 

 completely divided; a new appai'atus is connected with the primary 

 shoulder-girdle, represented by the cartilage; this non-carti- 

 laginous part is formed of bones which primitively belonged to the 

 integument, and in the course of its differentiation up to the 

 Mammalia it plays an important part. 



We must therefore distinguish the secondary from the primary 

 shoulder-girdle. The latter is always cartilaginous in the Sturiones, 

 though various bony plates of the integument are developed on it ; 

 the two lower ones I have shown to be the clavicle and infra-clavicle, 

 and the two upper ones the supra-clavicles. In the primary thoracic 

 cartilage wider spaces ai-e developed from the canals which are found 



