In the dog and the cat it also arises from the sacro-sciatic ligament. At this level its contour is distinguishable from that which corresponds to the gluteal muscles, so that we there find two prominences one above the other. The superior is formed by the gluteal muscles; the inferior corresponds to the tuberosity of the ischium. The two prominences are separated by a depression, from which the biceps emerges. We draw attention to this form, the character of which is so expressive of energy in the carnivora. In these animals the biceps is inserted, by its anterior fibres, into the articulation of the knee, while in the rest of its extent it covers in great measure by its aponeurosis the external aspect of the leg. In the pig, the biceps is but slightly marked off from the posterior part of the great gluteal. In the ox, the division between these two muscles is a little more distinct. In the horse, the sciatic origin of the biceps is covered by the semi-tendinosus, so that it only becomes free lower down, to appear in the space limited behind by the semi-tendinosus, and in front by the posterior part of the gluteus maximus. When the biceps contracts, taking its fixed point from above, it flexes the leg and helps to extend the thigh. If, on the other hand, it takes its fixed point from below, it lowers the ischium, makes the pelvis undergo a see-saw movement, and acts thus in the movement of rearing. It is sometimes called, on account of one of its actions, and the position which it occupies, the 'external flexor, or peroneal muscle of the leg.' =Semi-tendinosus= (Fig. 68, 31; Fig. 70, 37; Fig. 87, 1; Fig. 88, 1; Fig. 89, 28).--This muscle forms the contour of the thigh posteriorly, so that when the latter is viewed from the side, it is the semi-tendinosus above all that forms the outline. But, as we shall soon see, it is in this case more distinct above than below, because of the deviation which it undergoes in order to occupy by its inferior part the internal side of the leg. In the dog, the cat, and the ox, the semi-tendinosus arises from the tuberosity of the ischium only, as in the human species. In the pig, it also takes origin higher up from the sacro-sciatic ligament and the coccygeal aponeurosis. In the horse, it extends still further, for it is also attached to the crest of the sacrum.