1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 707 



externi of these ribs': — that the sternum of birds has undergone an 

 anterior shortening, consequent on that posterior translation of the 

 shoulder-girdle which is at once expressed by the lengtiiening of the 

 neck and the shortening of the trunk in the Avian as compared with 

 the Reptilian type ; owing to this change of place, the front part of 

 the sternum has been severed from the ribs that formed it, so that 

 these are now atrophied, and only traced in the embryo. 



It can scarcely be doubted that the manubrium sterni of mammals 

 owes its origin to ribs lost in a similar shortening of the trunk, by 

 which the seven constant vertebrae of the mammalian type established 

 their cervical character, ribs which are still present in the Mono- 

 tremes. The part of the Avian sternum called by Huxley the 

 " manubrium " or " rostrum " has, however, nothing in common 

 with the manubrium sterni of mammals ; it is a secondary outgrowth 

 formed for the attachment of the sterno- clavicular ligaments. The 

 anterior portion of the sternum of the Ostrich, however, is truly a 

 kind of manubrium ; it projects anteriorly between the attachments 

 of the coracoids, and, as is seen in the diagram (Plate XLII. fig. 3), 

 it projects far in front of the first sternal rib. 



2. Conclusion from the atrophy observed to take place of one or 

 two posterior ribs, and the addition of the median and posterior 

 part seen to take place in the embryo of the Ostrich : — that the 

 posterior shortening of the sternum suggested by Gegenbaur is but 

 slight, varying in different types ; and that it is far more than com- 

 pensated by the addition of the long metasteruum. 



3. Conclusion from the development observed, in connection with 

 changes in the muscles : — 



That the posterior lateral processes, though primitively represent- 

 ing the ends of the costal bands, are elongated and modified in 

 various tyj)es according to the exigencies of mechanical strain. For 

 this reason they are never found in connection with posterior ribs 

 that suffer atrophy, as might at first sight have been expected from 

 comparison of the costal processes of the sternum of Iguana. 



The great variation in form of the processes leads us to look 

 for some modifying cause that varies in the types to which they 

 belong. Now since the posterior border of the sternum affords 

 attachment to two opposite sets of muscles, (a) the pectoral, (J) the 

 abdominal, the resultant of their forces must be to some extent 

 expressed in its shape, for in general the outline of a bone tends to 

 express the direction and strength of the mechanical forces acting on i( . 

 In other words, we should expect to find the posterior border of the 

 sternum varying with the habits of the bird, whether it is a good flyer, 

 and uses its pectoral muscles most, or is accustomed to run or hop, 

 and thus makes a greater proportional use of its abdominal muscles. 

 This is exactly what takes place : in birds of great power of flight 

 the processes become long, broad, and finally confluent, so that a long 



^ It is understood that the m. longus lateralis cervicis (Gadow, Bronn, 

 Vogel, ]). 117), i. e. m. longus colli cxternus of Watson, seen in tlie Spheniscidse 

 [pp. cit. p. 61), is a continuation and serial homologue of the mm. intertrans- 

 versarii and intercostales externi, 



