Note 141 — Chapter II · THE ANATOMY OF A BIRD.[138]

The presence or absence of it, or of the other muscles, is used as a means for arranging the smaller divisions of the larger groups into which the two first-named sub-classes are, by the aid of other anatomical facts, divided. One striking advantage of this system, as suggested by the late Prof. A. H. Garrod, is that the characters of the ambiens have been observed to go hand in hand with certain other characters. Thus, the cæca found at the end of the small intestine are always present in the Homalogonatæ, or birds having the normal arrangement of knee-muscles; but in this connection there is another structure to be mentioned, namely, the so-called oil-gland, or gland by the secretion of which the bird “preens” its feathers, and which is always set in the skin in the region of the tail. Now this “uropygial,” or oil-gland, may or may not be provided with a tuft of feathers, and as there may or may not be cæca to the intestine, it follows that—(1) the gland may be tufted and there may be cæca, or (2) the gland may have no feathers and cæca may be present, or (3) there may be no cæca and a tufted gland, or (4) there may be no cæca and no tufts (the possible arrangement of neither being present is found in a few Pigeons). But this is not the place to follow out the details of this classification.

With regard to the proposition made by a French observer, M. Alix, that birds should be divided into the Homœomyarii, Entomyarii, and Ectomyarii, according to the character of certain of the flexor muscles at the back of the leg, it seems only necessary to remark that so far anatomical investigations have not supported his views, while his system would separate birds which seem to be closely allied.