XVI.] GENERAL CHARACTERS. 257 



may be in a very rudimentary condition, or altogether 

 suppressed. If one is absent, it is most commonly the 

 first. 



Excepting the Cetacea, no Mammals have more than three 

 phalanges to each digit, but they may occasionally have 

 fewer by suppression or ankylosis. 



The first or radial digit (also called pollex) is an exception 

 to the usual rule, one of its parts being constantly absent, for 

 while each of the other digits has commonly a metacarpal 

 and three phalanges, it has only three bones altogether. 

 Whether the missing one is the metacarpal or one of the 

 phalanges, is a subject which has occasioned much dis- 

 cussion, but has not yet been satisfactorily decided. In 

 accordance with the most usual custom, the proximal bone 

 of this digit will here be treated of as a metacarpal.^ 



The terminal phalanges of the digits are often specially 

 modified to support the nail, claw or hoof, and are called 

 " ungual phalanges." 



Very frequently a pair of small sesamoid bones are 

 developed in connection with the tendons passing over the 

 palmar surface of the articular heads of the metacarpals and 

 phalanges, and occasionally (as in the Armadillos) a larger 

 bone of similar nature is met with in the middle of the same 

 surface of the carpus and metacarpus. More rarely similar 

 bones occur on the dorsal surface of the phalangeal articu- 

 lations.2 



Each of the carpal bones ossifies from a single nucleus. 

 The metacarpals and phalanges have each a main nucleus 

 for the greater part of the bone, and usually an epiphysis at 



1 See Allen Thomson " On the Ossification of the First Metacarpal 

 Bone." {you7'n. Anat. and Physiology, Vol. III. p. 131.) 



2 These are almost always lost in prepared skeletons, but they occur 

 constantly in the common dog. 



S 



