of the Skull and the Skeleton. 851. 



influence of the muscles, it is obvious that those muscles which 

 are inserted at the extremities of bones must exercise a powerful 

 influence on the formation of epiphyses. Therefore epiphyses 

 and processes are to be looked for wherever the pressure and 

 tension on a bone become more than sufiicient to continue ossi- 

 fication. Now just as ossified epiphyses are not to be found in 

 bones where the pressure at the ends is small, so it would be 

 expected that in cases where the pressure and tension of the 

 bone is almost entirely at the ends, and the shaft does not 

 support the animal, the epiphyses should be enormously large, 

 while the shaft would be small. And in Plesiosaurs this is 

 actually found to be the case ; for the large limbs, swimming 

 powerfully through the yielding water, have experienced an 

 enormous lateral tension at the ends of the long bones without 

 any greater pressure in the direction of length. And therefore 

 it happens that the ends of the epiphyses which are attached to 

 the shaft become conical and penetrate down the girdling shaft 

 till they meet in the middle of the bone ; and, as might be anti- 

 cipated, that of the distal end is much the larger one. There- 

 fore it would seem possible, if the muscles attached were small, 

 and the bones so placed as only to experience tension and no 

 direct pressure, that the shaft might altogether disappear, and 

 only the two epiphyses remain, as I am inclined to suggest 

 may be the case with the bones which are called tarsal and 

 carpal — a conclusion to which I am led by a consideration of 

 the bones called the tarso-metatarsus in birds, which may be a 

 case in which the tarsus does develope a shaft ; and if so, then 

 the metatarsals, like the phalanges, as is usual in the other 

 Sauropsida, will be applied to the ground. There can be no 

 a priori reason for supposing that the tarsals and metatarsals 

 should unite together to form one bone; and all the facts of 

 osteology point to their remaining separate ; while an erect po- 

 sition for the metatarsal bone in a clawed animal is unusual, 

 and only partial even in jumping jerhoas, which it characterizes. 

 The careful dissections of the leg in the ostrich and crocodile 

 &c. by Dr. S. Haughton enable me to add a little evidence from 

 the muscles. The gastrocnemius muscle in the crocodile, as is 

 usual, is inserted in the os calcis (and tarsal bones). It weighed 

 0*14 oz., while the tibialis anticus and extensor digitorum com- 

 munis weighed 0"11 oz. But in the ostrich the gastrocnemido- 

 solseus is inserted into the middle of the so-called tarso-meta- 

 tarsal bone, and weighs 115^ oz., while all the other muscles of 

 the limb and those attached from it to tlie body only weigh 

 220 oz., the tibialis anticus and extensor digitorum communis 

 weighing 14 oz. Now there is nothing to induce us to expect 

 that the gastrocnemius would be inserted in the metatarsal hone, 



