We will present the reader here with a table showing the relative lengths, etc., of some of the bones we have thus far examined, in order that a study of their comparative development may be made. (The measurements are given in centimeters.)
| Sternum | ||||||
|---|---|---|---|---|---|---|
| Species. | Length from bifurcation of manubrium to end of body. | Depth of keel. | Humerus. | Radius and ulna. | Hand. | Long axis of skull. |
| Cinclus mexicanus. | 2.7 | 0.8 | 2.2 | 2.6 | 2.6 | 4.4 |
| Siurus nævius. | 1.9 | 0.6 | 1.7 | 2.1 | 1.7 | 3.1 |
| Salpinctes obsoletus. | 1.6 | 0.5 | 1.7 | 2.0 | 1.7 | 3.6 |
| Oreoscoptes montanus. | 2.3 | 0.7 | 2.2 | 2.7 | 2.4 | 4.2 |
| Sialia mexicana. | 2.3 | 0.8 | 2.0 | 2.8 | 2.3 | 3.5 |
| Anthus ludovicianus. | 2.1 | 0.7 | 1.8 | 2.5 | 2.1 | 2.9 |
| Merula migratoria. | 3.4 | 1.1 | 2.9 | 3.4 | ||
| Hesperocichla nævia. | 3.0 | 1.6 | 2.5 | 3.1 | 3.1 | 4.6 |
A great many points of extreme interest and of the highest importance reward the ornithotomist’s study of the pelvic limb of Cinclus; some of these the writer has already remarked upon in papers now in press, but he offers them here again, confident of the fact that they will be of interest to ornithologists generally, particularly to those whose aim it is to pursue the study more than “skin deep.”
In the adult Dipper the pelvic limb, as far as its skeleton is concerned, is made up of the most usual number of bones; the thigh having the femur, the leg the tibia and fibula, a patella, the tarsus the bone tarso-metatarsus, and finally a foot arranged upon the plan of four toes, with first, second, third, and fourth digit composed of 2, 3, 4, and 5 joints respectively.
I have already said that these bones are non-pneumatic, they are also of lengths proportionate to the size of the bird, the claws being curved about as much as they are in a typical Thrush. Anatomists have described certain general points for examination on these long bones composing the leg; many of these are present, but we shall only call the student’s attention to a few of them, so as to make clear what we have to point out hereafter. Nothing of striking variance marks the femur, as distinguishing it from the common form of the bone among birds of this class. The same might be said of the tibia, but we must note the two large flaring processes at the anterior and upper end of this, the larger bone of the leg; in this bone, too, the condyles are well developed below. The tarso-metatarsus, or the bone of the tarsus, we observe in the old bird, has rather a slender shaft, presenting for examination at its upper end the usual dilatation, crowned by a smooth, undulating surface to articulate with the tibia; behind this, at the same end, we find a tuberous process that has given comparative anatomists no little trouble to name; but we will speak of this further on. The lower end of the tarso-metatarsus has the little lateral facet for the diminutive first tarsal bone, and the three trochleæ for the other toes.
Let us now, after this brief survey of the bones in the adult take up the young of this species. We find first that the femur has grown in the usual manner, its lower end bearing the two large condyles has been formed by one epiphysis which included both of these articulate surfaces. Nothing of particular interest is to be observed in the development of the fibula or the small “splint bone” of the leg. The superior end of the tibia has been formed by the epiphysis including the two large processes that I spoke of above. These plates are called the procnemial and the ectocnemial processes, the inner and outer one respectively. They are turned slightly outwards, springing abruptly from the shaft in the adult, very much as I figured them in Lanius.
Such of my readers as have read my account of the development of Centrocercus in the Osteology of the Tetraonidæ, will remember what we had to say in regard to the lower end of the tibia and its growth, and also all that Professor Morse has done for us in that direction. The specimen we have of the young of Cinclus does not admit of the demonstration of the intermedium; the fibulare and the tibiale seem to ossify separately, however. We must admit, then, that in this instance we are no nearer solving the problem of the homologies of the avian tarsal segments than we were before, but a little light at least is thrown on the subject when we come to examine the next bone, the tarso-metatarsus.
In nearly all birds this bone has at the back part of its upper end a tuberous process, amalgamated with the shaft in the adult, that assumes various forms in different members of the class. This bony process has long been regarded with suspicion, as to whether it was one of the ankle or rather tarsal bones or not. Let us hear what a few of the authorities have to say in this matter. Professor Owen tells us in Vol. II of his Anatomy of the Vertebrates, when speaking in general terms of this process, that: “One or more longitudinal ridges at the back of the upper end of the metatarsal are called ‘calcaneal’; they intercept or bound tendinal grooves which, in some instances, are bridged over by bone and converted into canals; the ridges may be expanded and flattened.” This would lead one to think that the Professor might regard this process as the homologue of the os calcis, a tarsal bone.
Professor Huxley, in his Anatomy of Vertebrated Animals, page 254, tells us, in speaking of this process, that: “Again in most birds, the posterior face of the proximal end of the middle metatarsal, and the adjacent surface of the tarsal bone, grow out a process, which is commonly, but improperly, termed “calcaneal.” The inferior surface of this hypo-tarsus is sometimes simply flattened, sometimes traversed by grooves or canals, for the flexor tendons of the digits.”
Mivart says, when referring to birds: “Thus no projection corresponding with the tuberosity of the os calcis exists in this compound bone.” (Elementary Anatomy, p. 206.)