EXTERNAL PARTS OF BIRDS. — THE FEET. 127 



inner one. In nearly every three-toed bird, all three toes are anterior ; our single exception is 

 in the geuus Pkdides, where the true hind toe is wanting, the outer anterior one being reversed 

 as usual in zygodactyles. No bird has more toes beliiiid than in front. Birds' toes, and tluir 

 respective joints, are 



Numbered, in a certain definite order, as follows (see figs. 34, 36) : hind toe = first toe, 

 1^ ■ inner anterior toe = second toe, 2t ; middle anterior toe = third toe, 3/; outer anterior 

 toe = fourth toe, 4*. Such identification of It, 2t, St, it applies to the ordinary case of three 

 toes in front and one behind. But, obviously, it holds good for any other an-angement of the 

 toes, if we only know which one is changed iu position, — a thing always easy to learn, as we 

 shall see at once. Iu birds with the hind toe reversed, leaving all four in frout, the same 

 order is evident, though then 1 1 is the inner anterior, 2 t the next, etc. ; for it always happens, 

 when a liind toe turns forward, that it turns on the inner side of the foot. Similarly, iu yoke- 

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

 said ; then, evidently, inner hind toe = 1 * ; inner front toe = 2 * ; outer front toe = 3 ^ ; outer 

 hind toe = 4 i. In Trogonidce, with inner front toe reversed, the correction of the formula 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 1 is the missing 

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

 ization are afforded by two exotic genera of kingfishers, Ceyx and Alcyom, in which 2 t is 

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

 Tlie rule is proven by the 



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

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

 of Aves. In all birds, excepting Procellariidcs, 1 1 when present has two joints (not counting, 

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

 cdl birds, 3 1 has four joints. In nearli/ all birds, 4 1 has five joints. Thus, any digit has one 

 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 ^ by one, and of 4 f by one or ttvo. So when the joints do 

 not run 2, 3, 4, 5, for toes 1 to 4, they run 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 



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



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

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

 /^C^^/f^ us to tell what toes are missing in any case, or what ones are 

 xir^ t) out of the regular position. Thus, in Picoides, the hind toe, 

 ^ (P apparently 1 1, is known to be 4 t, because it is five-jointed ; 



^ in a troffon, the inner hind toe is 2 t, beinij three-jointed ; in 



Fig 40 Pha- o ; ^ 



langes of Cypse- t^*^ ostrich, with only two toes, 3* and it are seen to be 

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

 (See fig. 34, where the digits {ind their phalanges are numbered.) Besides 



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



- „ , r. 1 • • 1 ii Fig. 41.— Pha- 



some consequence in classification, resulting from their comparative lengths, langesotcaprimul- 



In some families of birds, one or more of the hasal 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, they generally decrease in leiiirth 



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



