chap, xiii.] DOUBLE-THUMBS : MAN. 349 



central digit on the presence of a digit external to it. If therefore it 

 be still called the "minimus" this term can only be applied to it by 

 virtue of its ordinal position.] 



For other cases of complete digits in this position see Auvard, Arch, de Tocoloqie, 

 xv. 1888, p. 633; Marsh, Lancet, 1889 (2), p. 739. 



(1) B. Single extra digit in other positions. 



Apart from cases of extra digit external to the minimus, cases of duplication of 

 the pollex or hallux (to be considered below), and cases of extra digits internal to 

 the pollex or hallux associated with change of symmetry of the digital series, the 

 remaining cases of single extra digit are very few. In other words, it is with digits 

 as with Meristic series in general, when a new member is added, the addition taking 

 place in such a way that homologies may be recognized, it is most often at one of 

 the ends of the series that the addition is made. Cases of extra digits in other 

 positions are in Man and Apes very rare, and even in some of the few recorded cases 

 of a new digit arising on the inner side of the minimus (No. 511) it should be re- 

 membered that this inner digit is judged to be the extra one rather than the outer 

 mainly by reason of its smaller size. I can only give particulars of few such cases, 

 and of the remainder no details are available. 



Simia satyrus (Orang-utan), having a rudimentary extra digit arising from the 

 internal side of the minimus of each hand: feet normal. In the left manus the 

 minimus has all joints moveable as usual ; the first phalanx is normal, but the 

 second is bent outwards nearly at right angles, thus making room for an extra digit 

 arising from the first phalanx and directed inwards. This digit is fixed and has no 

 articulation and no nail, but it is in its outer part bent back again towards the 

 minimus with which it is webbed. The structure in the right manus is almost the 

 same but the extra digit is larger and in its outer part free from the minimus, 

 bearing a nail. Bolau, Verh. naturw. Ver. Hamburg, 1879, N. F. in. p. 119. 



Woman : left pes bearing an extra digit articulating by an imperfect metatarsal 

 with outside of metatarsal of IV. The extra digit stands obliquely to the others, 

 sloping outwards and being attached by ligaments to the normal V. [The Cata- 

 logue states that the extra digit resembles a light digit, but I see no sufficient 

 evidence of this.] C. S. M., Ter. Cat. 312. 



[A case perhaps similar to foregoing is briefly quoted by Gruber, I. c, p. 471, 

 note 83, as being in the Vienna Museum of Anatomy.] 



Child : left metacarpal IV bore a supernumerary digit on external side. This 

 digit was shorter than the digit IV and was completely webbed to it. Broca, 

 quoted by Fort, I.e., p. 66. 



Foetus (otherwise abnormal) : left hand bore extra digit attached by peduncle to 

 first phalanx of digit IV. The minimus was separated from IV by a metacarpal 

 space, standing almost at right angles to it. Hennig, Sitzb. natnrf. Ges. Leipzig, 

 1888. Oct. 9. 



[Ammon (Die angeb. Krankh. d. Mensch. p. 101, PI. xxn. fig. 7) describes a case of 

 rudimentary finger appended to the "ring-finger" and is so quoted by Gruber; but 

 the figure apparently represents the appendage as attached to the minimus.] 



(2) Dtiplication 1 of single Digits, especially of the Pollex and Hallux. 



f$' Duplication of the pollex or of the hallux is one of the commonest 

 forms of polydactylism and numerous cases have been described by all 

 who have dealt with the subject. It consists in the development of 

 two digits, complete or incomplete, in the position of the usually single 

 series of bones composing the pollex (or hallux). In the section dealing 

 with polydactylism associated with change of Symmetry (p. 326) we 

 saw how upon the appearance of an extra digit in this position the 

 thumb itself may have three phalanges. In these cases the extra digit 

 may properly be considered as arising in Successive Series with the 



1 A few cases are thought by some to shew triplication of digits, but it seems 

 doubtful whether there is a case of division of one digit into three really equivalent 

 digits, perhaps excepting the thumb of No. 521. 



