THE MEMBERS. 195 



the posterior left, G, on the contrary, goes to rest on the ground, H'G, at a distance, 

 DG, equal to a half-step. 



While the latter, G, passes through its phase of contact, the centre of the 

 croup passes through the space from H' to H" and describes the arc H'H" equal 

 to that described by the right, because, in the normal gait, the steps are equal. 



Arrived at If", the left member, G, becomes elevated, H"G ; the right, on 

 the other hand, is now placed on the ground, H"D', to renew its phase of contact 

 at a distance, GD / , equal again to a half-step. 



During its progression, the right foot, therefore, proceeds from its initial po- 

 sition D to attain its final position D / . It consequently passes through the arc 

 Diy, while the centre of the croup only passes over the space H'H", which is 

 exactly one-half of the distance DD' '. 



H'H" is parallel to DD', for the isosceles triangles GH'D and GH"D' are 

 equal, since GD and GD' are equal. Again, the analogous angles GDH' and 

 D'GH" being equal, the lines DH' and GH" are parallel. 



Therefore, H'H" = DG = ^-> 



When one of the members has passed successively through the two 

 phases of contact and of elevation, the body effects what is called a 

 complete step, the two pairs of members (anterior and posterior) being 

 jointly concerned. During this evolution it can be seen that the centre 

 of movement, H, and consequently the centre of gravity, has progressed 

 from Hto H", or, what is equivalent, through a space which is equal to 

 the line DD', equal to the distance passed over by one of the feet, a 

 distance completed in two attitudes, HD, H"D', identical and successive. 



It follows from these statements that the length of the step will be 

 measured by the separation of DD', comprised between the successive 

 imprints left upon the ground surface by the same foot. 



But for an accurate analysis (such as is often necessary) the division 

 of the step into the two principal phases of contact and elevation, above 

 mentioned, is insufficient. It is necessary to subdivide each into a number 

 of equal secondary divisions called periods. All veterinary writers, 

 with Captain Raabe, recognize at present the following six periods (see 

 Fig. 55) : 



f 1st period, from to 1 ... Commencement of the contact. 

 Phase of contact, -j 2d period, from 1 to 2 ... Middle of the contact. 



(. 3d period, from 2 to 3 ... Termination of the contact. 

 Phase of elevation ( 4th peri d ' fr m 3 to 4 Li f tin 9 of the foot. 



or of extension. ] ^ peri d ' fr m 4 tO 5 ' ' ' Mddle f elevation ' 

 V. 6th period, from 5 to 6 ... Resting of the foot. 



Systematic Analysis of the Play of the Members. The 

 general scheme of the evolution of the members, which we have 

 given, has permitted us to prove a certain number of facts, and to 



