38 MATERIALS FOR A MEMOIR OX 



hind foot alternates with the fore foot of tlie same side lateral 

 heterochiry occurs. When with the fore foot of one side the hind 

 foot of the opposite side alternates, diagonal heterochiry takes 

 place. 



In connection with the terms flexion and extension the following 

 will be used : The movement of a limb against the medium in 

 "which the animal is moving constitutes the stroke. The move- 

 ment in preparation of the stroke constitutes the recover. In the 

 description of the " hand-over-hand" movements of the sloth and 

 the ape, the word " flexion" has no place ; yet the " recover" is 

 used in as exact a sense as in the movements of any other animal. 



When a limb of a terrestrial quadruped rests upon the ground 

 it may be said to be /'on," and when not on the ground, to be 

 " off"." 



The term sura "svill be employed as a convenient word to in- 

 clude the hind limb from the knee to the ankle. Cms has been 

 retained so generally as a synonyme for the entire posterior ex- 

 tremity as not to be available. 



" Stroke" is the period of impact. It is included in flexion, and 

 constitutes its first stage. " Recover" embraces the last stage of 

 flexion and the whole of the period of " extension." The terms 

 '"' stroke" and " recover" are by no means the same as flexion and 

 extension. They simply express certain phases of linib-functiou 

 which are seen during the acts of flexion and extension. 



The support of the trunk begins in advance of the neck — viz., 

 at or in front of the occiput (see p. 54) — and passes back to a 

 point midway between the withers and the rump. In the camel 

 the fore foot supports the trunk at a point as far back as the mid- 

 dle of the posterior dorsal hump. In the same animal the pos- 

 terior extremity supports the body from the dorsal concavity 

 backward. (See p. 52.) 



In the Canadian deer (series 695, Fig. 1) and in the fallow 

 deer (series 682 B, Figs. 2 and 3) the fore limb at one stage of the 

 gait is directly under the trunk, and is the only support of the 

 body. In Procyon (series 744, Fig. 3; see also p. 72) two limbs 

 (the right fore limb and the left hind limb) are in the same posi- 

 tion as the foregoing. In the sloth (series 750, Figs. 1, 6, 7, 8, 9, 

 and 12; see also p. 7S) the two lateral limbs are also the same. 



