LOCOMOTION OF MAMMALIA. 65 



bones, or under ligaments, by which the muscles are capable of 

 producing effects on distant organs without disturbing the sym- 

 metry of the body, an effect which, owing to the limited power 

 of contraction in the muscles, could 



O 4 



be accomplished in no other way. 



The joints in the mammalian 

 skeleton are chiefly of two kinds, 

 ( ginglymoid ' or hinge-joints, and 

 6 enarthrodial ' or ball-and-socket r" 

 joints. In Man the former are less 

 definitely fitted for motion on one 

 plane than in most brutes. The 



Lever of the third kind. 



arm and fore-arm move in concen- 

 tric planes upon the elbow-joint ; the knee-joint allows a certain 

 rocking motion of the leg upon the thigh ; the ankle-joint has a 

 greater latitude of motion, and the foot may be directed out of 

 the plane of the leg's motion. 



Atmospheric pressure exercises its influence upon joints. Dr. 

 Arnott estimates the amount of that on the knee-joint at 60 Ibs. ; 

 AVeber of that on the hip-joint at about 26 Ibs. : in the hip-joint 

 of the Megatherium the pressure could not have been less than 

 150 Ibs. 



A. Swimming. - - Quadrupeds with inflated lungs are of less 

 specific gravity than water, and swim by alternate extension and 

 flexion of their legs; the effective stroke being the act of extension, 

 when the limb presents a larger area to the water than in flexion : 

 this is seen in the Horse, which strikes the water with the ex- 

 panded and subconcave surface of the hoof, but draws the convex 

 conical part through the water in the bending of the limb pre- 

 paratory to the next effective stroke. In the best water dogs the 

 digits are connected by webs, which are stretched in the back or 

 down-stroke, folded in the return movement. The feet of the 

 Otter are broader, especially the hind ones, and more fully palmated. 

 The Seals and Whales have the limbs fashioned as fins. 



Man, Avith the chest well expanded, is lighter than water : the 

 presence of mind which counteracts the tendency produced by 

 immersion in a cold and dense medium to expel the air from the 

 lungs is the first safeguard against drowning ; and next, if the 

 art of swimming has not been learnt, to keep the head immersed 

 to the mouth and nose, and to refrain from the misdirected 

 struggles of terror which tend only to hasten on the catastrophe. 



In swimming, the hands and feet are employed so as to present 

 the greatest surface to the water in the effective stroke, the least in 



VOL. III. F 



