840 A MANUAL OF PHYSIOLOGY 



comes down, the heel rises, the leg is extended, and, with a parting 

 push from the toe, the leg again swings free. By this manoeuvre the 

 body is raised vertically, tilted to the opposite side, and also pushed 

 in advance. 



The forward swing of the leg is only slightly, if at all, due to 

 muscular action ; it is more like the oscillation of a pendulum dis- 

 placed behind its position of equilibrium, and swinging through that 

 position, and in front of it, under the influence of gravity. For this 

 reason the natural pace of a tall man is longer and slower than that 

 of a short man ; but it may be modified by voluntary effort, as when 

 a rank of soldiers of different height keeps step. 



The lateral swing of the body is illustrated by the everyday 

 experience that two persons knock against each other when they 

 try to walk close together without keeping step. In step both swing 

 their bodies to the same side at the same moment, and there is no 

 jarring. 



Even in the fastest walking on level ground there is a short time 

 during which both feet touch the ground together, the one leg not 

 beginning its swing until the other foot has begun to be set down. 

 In running, on the other hand, there is an interval during which 

 the body is completely in the air, while in walking uphill or in carry- 

 ing a load the one foot is not raised until the other has been firmly 

 planted. 



Functions of the Cerebral Cortex. When an animal, like 

 a frog, is deprived of its cerebral hemispheres, the power of 

 automatic voluntary movement appears to be definitively and 

 entirely lost. The animal, as soon as the effects of the anaes- 

 thetic and the shock of the operation have passed away, draws 

 up its legs, erects its head, and assumes the characteristic 

 position of the normal frog at rest. So close may be the re- 

 semblance, that if all external signs of the operation have been 

 concealed, it may not be possible for a casual observer to tell 

 merely by inspection which is the intact and which the ' brain- 

 less ' frog. The latter will jump if it be touched or otherwise 

 stimulated. It will croak if its flanks be stroked or gently 

 squeezed together. It will swim if thrown into water. If 

 placed on its back, it will promptly recover its normal position. 

 But it will do all these things as a machine would do them, 

 without purpose, without regard to its environment, with a kind 

 of ' fatal ' regularity. Every time it is stimulated it will jump, 

 every time its flanks are squeezed it will croak, and, in the 

 absence of all stimulation, it will sit still till it withers to a 

 mummy, even by the side of the water that might for a while 

 preserve it. 



A Necturus, without its cerebral hemispheres, will, like the 

 frog, refuse to lie on its back. On stimulation it moves its 

 feet or tail, or its whole body ; but if not interfered with, it 

 lies for an indefinite time in the same position. Its gills are 

 seen to execute rhythmic movements, which never stop, and 

 rarely slacken, except for an instant, when some part of the 



