14 NATURAL HISTORY OF BIRDS. 



ratio of the others decreasing gradually toward the ends. By progressive specializa- 

 tion the number of phalanges is often reduced, e.g., in some swifts which have only 

 two, three, three, and three phalanges respectively, and the proportion of their length 

 modified. 



Concerning the muscles of birds, we should like to enlarge upon the mechanism 

 moving the wings, and that wonderful arrangement by which the toes of the perching 

 bird are automatically kept in a grasping position by means of the bird's own weight 

 alone ; but want of space permits us only to mention and explain a few technical terms 

 and signs which will be found useful further on. The late Professor Garrod used the 

 letters A, B, X, and Y to represent certain muscles of the thighs which he considered 

 to be of particular taxonomic value, viz., respectively, the femoro-caudal, the accessory 

 femoro-caudal, the semi-ten din osus, and the accessory semi-tendinosus ; thus, saying 

 that the muscular formula of the secretary bird is BXY means that the three latter 

 are present. The formula A in the falcon means that none but the first-mentioned is 

 to be found. 



Besides the above, two other muscles belong to the femoral region of birds, the 

 account of which we shall give as nearly as possible in the late Professor Forbes's words, 

 viz., the glutens primus and the ambiens. The former is, as a rule, not small, and is only 

 seldom absent, e.g., in the Bucerotidae and Palamedeidae ; the latter, the ambiens, lies 

 on the lower or inner surface of the thigh. As generally developed, it is a more or 

 less slender fusiform muscle, which, arising from the praepubic process of the pelvis, 

 close in front of the acetabulum, runs along the inner side of the thigh superficially, and 

 then, as a thin tendon, over the bend of the knee in some cases perforating the 

 patella to the outer side of the leg, terminating thereby joining one of the tendons 

 of the superficial flexor of the toes. In all passerine birds, and some others, it is 

 always absent; these arc termed anomalogonatous birds, in contradistinction to the 

 more generalized types which are homologonatous, and we denote the presence or 

 absence of the ambiens muscle by the signs -(- or . 



In all birds there are two deep flexor muscles of the toes, the tendons of which 

 run along the posterior (plantar) aspect of the metatarsus, one the deep flexor of the 

 first toe (f. longus hallucis), the other closing the remainder of the toes, flexor per- 

 forans digitorum, the former being always external to, or superficial of, the latter 

 when passing the metatarse. In all Passeres and a few other birds, e.g. Upupa, these 

 tendons are quite independent of each other, so that if the flexor of the first toe be 

 artificially pulled, no closing (flexion) of the other ones takes place. In all other birds, 

 however, the two tendons are more or less intimately connected by a fibrous band 

 (vinculuni), or may even completely blend. 



By far the most interesting feature, however, brought out by Prof. Garrod's inves- 

 tigations into this subject is the discovery of the existence of the entirely different 

 types of plantar arrangement in the so-called zygodactyle birds (with usually two toes 

 in front and two turned backward), as well as the fact that the diversity of type 

 exactly coincides with the two groups of birds so marked out, being respectively homolo- 

 and anomalo-gonatous. Thus in the parrots, cuckoos, and Musophagidse, which all 

 belong to the former group, the plantar tendons are distributed in exactly the same 

 way as in the common fowl, the flexor perforans supplying second, third, and fourth 

 digits, and the./*, hallucis the first digit alone. In all the anomalogonatous zygodac- 

 tyle birds (all of which lack the ambiens and accessory femoro-caudal muscles) namely 

 the Picidoe, Capitonidae, and their allies, Bucconida? and Galbulidaa, an entirely unique 



