THE PECTORAL LIMB 



was a tendinous band stretching from the humeral head to the olecranon 

 to which I referred as a triceps ; but there was nothing else distad except 

 fibers identical with those strengthening the tissue between the digits. 

 In Monodon I distinguished and tentatively named by virtue of their 

 position a triceps, biceps relic, flexor digitorum and extensor digitorum, 

 but the two latter were rather loosely associated fascicular bundles and I 

 cannot even be entirely sure without histological preparation that they 

 really contained muscle fibers. It is thus indicated that at least in a great 

 many odontocetes the intrinsic musculature of the flipper proper is 

 considerably more atrophied than in mysticetes, and is on the rapid road 

 to total disappearance. 



As mentioned there is a tendency in the Cetacea toward flattening of 

 the antibrachial bones, and this is especially pronounced in those sorts 

 in which this segment is the shortest. Like the case of the sea-lion this 

 is entirely expected, for the antibrachium is a mechanical part of the 

 flipper and is subjected to the same influences as those which have oper- 

 ated to shape the manus. Also like the sea-lion there appears to be need 

 for an antibrachium that is externally broader than the breadth of bone 

 can furnish, and there is disposition of tough fibrous tissue not only on 

 the radial border, but softer tissue in more generous amount upon the 

 ulnar border as well. As previously mentioned the tendency in most 

 sorts of odontocetes at least would seem to be toward the reduction of 

 the radius and ulna to the size and conformation of the larger carpal 

 elements. 



In considering the manus of aquatic mammals we come to the critical 

 part of the fore limb. All other parts of the arm are directly influenced 

 by it or by certain body factors; hence the manus may be expected to 

 reflect in most perfect degree the stimuli that have operated upon the 

 pectoral appendage. In no two diverse sorts of aquatic mammals is this 

 detail alike, for the reasons that in no two sorts has it been used in 

 exactly the same manner, and that diverse kinds of aquatic mammals 

 split off from the main stems and have constituted individually separate 

 lines, each developing its own peculiarities, since before the manus be- 

 came so highly specialized. In consequence it is unsafe to venture any 

 broad generalities except that if the manus be used as a propulsive organ 

 or an equilibrator it can be expected eventually to assume the character- 

 istics of a paddle or flipper in which the bony element's become flattened 

 and simple, without movable joints. Usually there will also be apparent 

 a stimulus for a lengthening of the manus at the expense of the more 

 proximal brachial elements. 



[239] 



