OCR Text |
Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 707 externi of these ribs1:-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 lengthening 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 vertebras 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 compensated by the addition of the long metasternum. 3. Conclusion from the development, observed, in connection with changes in the muscles :- That the posterior lateral processes, though primitively represent-iu°- the ends of the costal bands, are elongated and modified in various types 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, (b) 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 it. 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 1 It is understood that the m. longus lateralis cervicis (Gadow, Bronn, Vogel, p. 117), i- e. m. longus colli externus of Watson, seen in the Spheniscidas {opfcit. p. 61),' is a continuation and serial homologue of the m m . intertrans-versarii and intercostales externi. |