[OSTEOLOGICAL COLLECTION]
ILLUSTRATIVE OF THE MODIFICATIONS OF THE
ORNITHOSAURIA (OR PTERODACTYLES) IN THE
CAMBRIDGE UPPER GREENSAND.
| Case. | Comp. | Tablet. |
| J | a | 1 |
Pectoral Girdle.
STERNUM.
[Pl. 1, fig. 1.]
The Sternum is the key to the bony apparatus supporting the anterior limbs. In the Pterodactyles from the Cambridge Greensand it has been well figured and described by Professor Owen, who enunciated its resemblance to the sternum of birds. The sternum in Pterodactyles from the Lithographic Slate, shows its proportional size to the body. The examples found in the Cambridge Greensand have as yet shown no evidence of a composite character like that attributed to Rhamphorhynchus Gemmingi.
The sternum consists of an expanded symmetrical shield having its lateral halves, which are inclined to each other at a large angle (about 150°), contracted superiorly, behind and immediately below the synovial cavities for the coracoids. The vertical angular ridge in which the lateral portions of the sternum unite becomes elevated as it is followed anteriorly, into a strong keel. This keel or interpectoral process is highest in front of the articulations for the coracoids; but the degree of elevation varies with the species. It is prolonged upward and in front of the coracoids for some distance, becoming very massive, and the prolonged mass which is flattened from side to side, reaches laterally to the outer margins of the coracoid articulations, and on the visceral side a little between and over them. The anterior crest of the keel shows the attachment of powerful muscles.
Professor Owen has observed that only in birds are distinct synovial cavities provided for the coracoids, and that no reptile has a sternum showing characters like those seen in the Pterodactyle. These coracoid cavities are placed as in birds, close together, behind the manubrium, which forms the hindermost part of the keel. They are convex transversely, concave from front to back as in birds, and look upward at an angle of 35°, their main direction being outward and a little backward. Professor Owen recognises the function of the shield-shaped sternum in relation to the mechanism of respiration on the one hand, and on the other hand, for the attachment of pectoral muscles of great bulk and strength.
As is well known, the muscles of the breast in most birds consist chiefly of the 1st, 2nd, and 3rd pectoral muscles, and the coraco-brachialis.
The peculiar form of the bird's sternum appears to be due to the vertical development of the second pectoral muscle, since when the 1st and 3rd muscles are dissected off, the appearance presented nearly resembles that of the sternum in Pterodactyles. There can however be no doubt but that the third pectoral muscle, which in most birds is but feebly developed, attained a far greater bulk in the Pterodactyle, because there is evidence of its powerful insertion in the distal anterior face of the coracoid, as well as of the great lateral extension of the sternal shield to which such a muscle must—by the analogy of birds—have been attached. The peculiar lateral emargination of the sternum appears to be due to the anterior sternal termination of this muscle, caused by the outward direction of the coracoid bone.