We mentioned above that the 'tendon' descends vertically from the carpus towards the fetlocks. This is as it should be. But, in some horses, it is oblique downwards and backwards, so that the canon, instead of being of equal depth from before backwards in its whole length, is a little narrower in its upper part. This results from the fact that the tendons of the flexors, too firmly bound by the carpal sheath, gradually separate as they pass from the metacarpus, going to join the fetlock; hence the obliquity pointed out above. This abnormality producing a deleterious result, in the sense that the tendinous apparatus acts with less strength as an organ of support, it constitutes a defect of conformation which is expressed by saying that the tendon has 'failed.' =Long Proper Flexor of the Thumb= (_Flexor longus pollicis_) (Fig. 76, 14).--As we have already pointed out, this muscle is represented in quadrupeds by the radial bundle of the deep flexor of the digits, so that the two muscles are in reality blended the one to the other. This union is sometimes found, but only as an abnormality, in the human species. We have met some examples of this in the course of our dissections. =Pronator Quadratus.=--This muscle conforms to the general law which we have already pointed out in connection with those which have for their action the rotation of the radius around the ulna. We remember, indeed, that when the bones of the forearm are fused with one another, the muscles which are destined to produce a mobility which has then become impossible disappear at the same blow. [Illustration: FIG. 81.--DIAGRAM OF THE POSTERIOR PART OF A TRANSVERSE SECTION PASSING THROUGH THE MIDDLE OF THE LEFT FORE-LIMB OF THE DOG: SURFACE OF THE INFERIOR SEGMENT OF THE SECTION. 1, Radius; 2, ulna; 3, posterior ulnar; 4, anterior ulnar; 5, great palmar (_flexor carpi radialis_); 6, flexors of the digits.] [Illustration: FIG. 82.--DIAGRAM OF A HORIZONTAL SECTION OF THE MIDDLE OF THE FOREARM OF THE LEFT LEG OF THE HORSE: SURFACE OF THE INFERIOR SEGMENT OF THE SECTION. 1, Radius; 2, ulna; 3, posterior ulnar; 4, anterior ulnar; 5, great palmar (_flexor carpi radialis_); 6, flexors of the digits.] For this cause we do not find the square pronator in either the ox or the horse, but can demonstrate its presence in the dog and the cat. It is very deeply situated. This is why, and also on account of the plan which we have traced for ourselves, we will simply say that it is situated on the postero-internal aspect of the skeleton of the forearm, and that it extends from the ulna to the radius. It seems to us, however, sufficiently interesting to add that, instead of occupying, as in the human species, the inferior fourth of the two bones, it extends, particularly in the dog, over their whole length, with the exception of their superior and inferior extremities. Muscles of the Hand