THE SKELETON 81 



month of fcetal life, giving rise to a prominence which can be 

 recognised in the adult. This prominence is present also in the 

 Chimpanzee, the Gorilla, and Hylobcdes ; and as the centrale is 

 most probably distinct in the embryo of these, it may well be 

 that in them, as in Man, its independent existence has not long 

 been suppressed. Further confirmation of this is afforded by the 

 fact that it is still an independent bone in 0'4 per cent of even 

 adult human beings, and that, normally, it retains its independence 

 in the Orang and in most Monkeys. 



In many Mammals (especially Marsupials, Rodents, and Insectivores) 

 cartilaginous or bony skeletal elements occur on the outer and inner borders 

 of both fore- and hind-limbs, which not only bear a superficial resemblance 

 to the digital skeleton, but may in some cases be clad, like the true digits, in 

 either a claw or a callous horny integument. Similar structures occur in 

 many of the lower Vertebrates (Reptiles and Amphibians). These organs 

 were formerly considered by both von Bardeleben and myself to be vestiges 

 of now vanished digits, and were named by us " praepollex," " prrehallux," 

 and " postminimus." 



I have, however, entirely changed my opinion as to the supposed atavistic 

 nature of these structures, and now agree with others that these "super- 

 numerary rays," whether they occur in the lower or the higher Vertebra ta, 

 are to be regarded rather as progressive structures of convergent and second- 

 arily adaptive significance. Baur has contended, before all others, that the 

 facts of palaeontology favour the view that the terrestrial Vertebrata never 

 possessed more than five rays in the skeleton of either fore- or hind-limb ; 1 

 and my own recent investigations into the development of the limb-skeleton 

 entirely confirm this conclusion. 



From this point of view, the condition of " hyperdactyly," which not ~*j 

 infrequently appears in Man and is often inherited for many generations, 

 loses its supposed atavistic significance. 



THE SKELETON OF THE HIND-LIMB 



The human femur usually bears at its head two processes for 

 muscular attachment, known as the trochanters, inasmuch as 

 they give insertion to the rotator muscles of the limb. Special 

 interest centres in the not infrequent presence of a, third trochanter 

 (&'"., Fig. 58), a development of the roughened area (tuberositas 

 glutealis) which occurs on the external border of the bone 



1 [It is an interesting corollary to this, that the only fossilised limb in which any- 

 thing comparable to a sixth digit has been found, is a fore-limb which, if not actually 

 Mammalian, is that of a Reptile with Mammalian characters (Tlicriodesmus, from the 

 Mesozoic beds of South Africa, cf. von Bardeleben, Proc. Zool. Soc., Lond., 1889, p. 

 259 ; and Seeley, Proc. Roy. Soc., Lond., vol. Iv. p. 227). Nor must it be forgotten 

 that the " prahallux " in its most highly differentiated and digit-like form (Frogs 

 and Toads) is cartilaginous, i.e. so constituted that it would not be preserved in the 

 fossil state.] 



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