EXTEBNAL PARTS OF BIRDS. ^ THE FEET. 127 



inner one. In nearly every tlirec-tfH'd bird, all three toes are anterior ; our single exception is 

 in the genus Picoides, where the true hind toe is wanting, the outer anterior one being reversed 

 as usual in zygodactyles. No bird has mfire toes behind than in front, liirds' toes, and their 

 lespective joints, are 



Numbered, in a certain definite order, as follows (see figs. 34, 156): hind toe = first toe, 

 1 t; inner anterior toe =^ second toe, 2t ; middle anterior toe = tJnrd toe, 3i; outer anterior 

 toe ^fourth toe, 4 1. Such identification of 1 t, 2 t, 'it, it applies to the ordinary ease of three 

 toes in front and one beliind. But, obviously, it holds good for any other arrangement of the 

 toes, if wo only know which one is changed in position, — a thing always e;isy to learn, as we 

 shall see at once. In birds with the liind toe reversed, leaving all four in front, the same 

 order is evident, though then It is the inner antericjr, 2 t the next, etc. ; for it always happens, 

 when a hind toe turns forvi'ard, that it turns on the inner side of the foot. SimDarly, in yoke- 

 toed birds (excepting Trogonidte), it is the outer anterior which is turned backward, as above 

 said ; then, evidently, inner hind toe = lt ; inner front toe = 2t ; outer front toe = 'it ; outer 

 hind toe = 4 f . In Trogonidce, with inner front toe reversed, the coiTecticjn of the fVjrmula is 

 easily made. Moreover, when the number of toes decreases from four to three or two, the 

 digits are almost always reduced in the same order : thus, in three-toed birds, 1 i is the missing 

 one ; in the two-toed ostrich, 1 1 and 2 1 are gone. The only known exceptions to this general- 

 ization are afforded by two exotic genera of kingfishers, Ceijx and Alcyone, in which 2 ( is 

 defective ; and by the anomalous passerine Cholornis of China, in which 4 t is in like case. 

 The rule is proven by the 



Number of Phalanges, or joints, (if the digits. The constancy of the joints in birds' 



toes is remarkable, — it is one of the strongest expressions of the highly monomorphic character 



lA' Aves. In all birds, excepting Procellariidce, 1 t when present has two joints (not counting, 



of course, the accessory metatarsal). In all birds, 2 t when present has three joints. In nearhj 



all birds, 3 t has four joints. In nearlij all birds, 4 t has five joints. Thus, any digit has <inc 



more joint than the number of itself. The exceptions to this regularity consist in the lessening 



of the number of joints of 1 i or 3 t by one, and of 4 < by one or two. So when the joints do 



not run 2, 3, 4, 5, for toes 1 to 4, they ran either, 1, 3, 4, 5, or 2, 3, 4, 4, or 2, 3, 3, 3. (These 



statements do not regard the anomalous cases of Ceyx, Alcyone, and Cholornis — see above.) 



This variability is nearly confined to certain Picarian birds : our examples of it are in certain 



genera of Cypselina, fig. 40, where the ratio is 2, 3, 3, 3, 



of Caprimulgina:, fig. 41, where it is 2, 3, 4, 4 ; and the petrel 



family, with 1, 3, 4, 5. Such admirable conservatism enables 



us to tell what toes are missing in any case, or what ones are 



out of the regular position. Thus, in Picoides, the hind toe, zJ-^J^ V\ 



a2)parently 1 1, is known to be 4 t, because it is five-jointed ; -Jv/ ll 



in a trogon, the inner hind toe is 2 t, beint; three-jointed ; in /, .y /; r. 

 Fig. 40.— Pha- , *^ , ' . , , „ , ^ ^ \ P // U 



langes of Cypse- the ostrich, with only two toes, it ami it are seen to be '^ </ rn 



line foot, 2, 3, 3, 3. preserved, because they are respectively four- and five-jointed. [j ^ 



(See fig. 34, where the digits and their phalanges are numbered.) Besides \^ 



this interesting numerical ratio, the phalanges have otlier inter-relations of 



some consequence in classification, resulting from their comparative lengths, lunges of Caprimiil- 



In some families of birds, one or more of the basal or proximal phalanges gine foot, 2, 3, 4,4, 



(those next to the foot — opposed to distal, or those at the ends of the digits) of the front toes 



are extremely short, being mere nodules of bone (fig. 40) ; in other and more frequent cases, 



they are the longest of all, as in figs. 34, 41. On the whole, tliey generally decrease in length 



from proximal to distal extremity, and the last one of any toe is quite small, serving merely 



