OCR Text |
Show 696 MISS B. LINDSAY ON THE AVIAN STERNUM. [June 16, (Spheniscidfe), partly overlying the pectoralis medius, a muscle described by Watson and named by him the Dermo-humeralis, which takes its rise from the head of the humerus, and forms in the abdominal region a thin band of longitudinal fibres lying near the median line : this is, in other words, a ventral part of the rectus continuous with a lateral part differentiated from the pectoralis. In these birds also the rectus is complex, consisting of two sheets (Watson opud Bronn). B. Middle sheet, most massive of the three, and attached to the posterior-lateral borders of the sternum, which apparently corresponds with the rectus of other birds. C. Thin inner sheet, which has the transversus attached to its edges. This takes attachment after passing inside the sternum for uearly one third the whole length of the latter. In the later stages its attachment travels backwards towards the end of the sternum, till in the embryo of 17 days it is completely united with the main body of the rectus, while the transversus has acquired its usual position with regard to the latter. This late ontogenetic change in the rectus seems to represent the posterior translation of the attachment of the rectus, during the late phylogenetic development of the long metasternum characteristic of this type, which apparently splits the muscle in two, and carries the upper part backwards as it grows. 3. In the earliest stage of the Guillemot, as in the Ostrich, there are seen transverse bands of muscle ; these are afterwards lost, and fused into a uniform supercostal sheet continuous with the obliquus externus. This supercostal sheet eventually, in the latest stages examined, acquires attachment to individual ribs, while at the same time the obliquus externus acquires an attachment to the ribs and lateral borders of the sternum ; the phylogenetic meaning of these changes is obvious. 4. There are indications of a slight posterior shortening of the costal sternum. There are 7 sternal ribs in the adult, but in the early embryo 9 ; of these the 7th, 8th, and 9th have no uncinate processes. Fig. I. (p. 690) shows an adult specimen in which the 9th rib is not only free at its sternal end, but atrophied also towards the dorsal region of its spinal parts. This mode of atrophy at both ends, which leaves a rudiment of the middle part of the rib, occurs again in the Chick. In the Ostrich, as has been seen, atrophy begins at or near the sternal end, and travels uniformly towards the spinal region. Neither in the Chick nor in the Ostrich do we find that marked division between spinal and sternal portions of the rib which in the case of the rudimentary 9th rib of the Guillemot divides it into two pieces. The allied species, U. brunnichii, may be compared with this ; the adult has also 7 sternal ribs, and 2 posterior ribs without uncinate processes, which do not reach the sternum. There is also some indication of anterior shortening of the sternum, for one of the early embryos shows the attachment of a rib, which in the adult (cf. Plate XLIV. fig. 9) falls short of the sternum. 5. The metasternum is remarkable for the great reduction of the |