TFIK MORSK IN MOTION. 115 



movements. It is only necessary, in order to determine the movements 

 of the several limbs, to suppose either of them to be right or left, and 

 follow it as such throughout the stride. Examples have been selected 

 of the two principal paces, when the horses were light-colored, to 

 reproduce in heliotype, a process which furnishes an exact tran- 

 script of the original photograph by the same agency, namely, 

 the sun. 



The walk is the simplest of the paces, and best understood. It is 

 defined to be that pace in which one foot is not raised until its fellow 

 is upon the ground. The definition is as applicable to quadrupeds as 

 to bipeds, if in the former we assume the two anterior and the two 

 posterior extremities as pairs. The slow walk, or saunter, represents 

 the pendulum of writers on animal mechanics, by whom the leg was 

 supposed to swing like a pendulum on its centre, but little muscular 

 force being used except to counteract the attraction of gravity. 



A man in walking throws the centre of gravity over the leg, which 

 is to serve, for the moment, as a column of support, and leans forward 

 until the centre of gravity is in advance of the foot as a base ; this ren- 

 necessary the advance of the other foot to serve as a new base, 

 and the action of the flexor muscles upon the toes, with the weight of 

 the suspended leg, carries the centre of gravity diagonally forward until 

 it is again supported by the other foot. These movements are all 

 detailed and formulated by the old writers, and are referred to here for 

 the purpose of bringing the science of animal dynamics, as it has 

 been taught until now, freshly to the mind of the reader. 



It must be conceded that we have advanced the science of animal 

 mechanics somewhat in this treatise, and demonstrated the fact that 

 its problems are not to be solved by physics, as heretofore attempted, 

 nor yet by vital force exclusively ; that animal motion in its highest 

 manifestation is the resultant of both, chiefly of vital force, but neither 

 can be ignored by one who would understand the subject. 



Each one of these elementary acts of progression is a step, and a 

 series of them is a walk. The walk of a quadruped is more complex 

 and perfect than that of a biped ; for while the latter is compelled to 

 oscillate his body in order to balance it upon each foot alternately, the 



