Differentiation. 285 



developed in those of the mammalia which have loose skins, as, for ex- 

 ample, the hedgehog, porcupine and porpoise. In the hedgehog, when 

 the muscles contract, the animal becomes rolled up as in a bag of mus- 

 cles. The sportive gambols of a school of porpoises are effected by an 

 abundant supply of these skin muscles; in the horse the skin muscle is 

 called the panniculus carnosus, and everyone who has seen a horse 

 twitching its skin to get rid of troublesome flies, will easily understand 

 how serviceable it is to that animal. 



' 'In all human beings there is a small muscle going from the coracoid, 

 a hooked process on the upper end of the shoulder-blade, to the inner 

 side of the arm-bone, about the junction of its upper and middle third. 

 Sometimes .this muscle is continued down to the lower end of the arm- 

 bone ; or, again, it may be quite short and attached to the bag of fibrous 

 tissue covering the shoulder-joint. On referring to the anatomy of the 

 lower animals it is found that both these varieties exist normally but in 

 a much more highly developed state ; they are especially well seen in 

 animals which use their fore-limbs for digging, climbing or swimming. 

 In them the muscle is of large size and reaches to the inner edge of the 

 lower extremity of the arm-bone. In man, when it reaches thus far it 

 is only rudimentary and of no use. Another muscle, which I have 

 seen in about three per cent, of human subjects, is a small one which 

 goes from the breast-bone to the upper end of the shoulder-blade. This 

 muscle is well developed in animals which have no collar-bones ; it 

 reaches its highest development in the horse, pig, hippopotamus and 

 elephant. It is also seen in the guinea-pig, Norway-rat and wombat. 

 It is quite rudimentary when it exists in man, and serves no useful 

 purpose. 



"In man, near the elbow-joint and lying close together, are two mus- 

 cles going from the upper to the lower arm ; one in front ( brachialis an- 

 ticus ) which helps to bend the elbow, and the other to the outer side 

 ( supinator longus) which supinates or twists the fore-arm outward. As 

 a rule these muscles are quite distinct, though they lie side by side ; but 

 in about one per cent, of cases they are joined together by muscular 

 fibres. This is the normal arrangement in apes and monkeys, the 

 union of these two muscles aiding them greatly in twisting their bodies 

 when hanging by their fore-limbs to the branches of trees. Again, in 

 apes the muscle forming the posterior fold of the arm-pit is always pro- 

 longed down to the prominence on the back of the elbow. In the long- 

 armed apes this muscle is especially well developed, and serves to swing 

 the whole arm rapidly and powerfully forward -a movement which is 

 of the greatest importance for dexterous^ grasping remote branches 

 while in the act of climbing. The same prolongation of this muscle is 

 occasionally seen in man, though in a much less developed state, and 



