In lean horses a deep groove separates the mass formed by the semi-membranosus and semi-tendinosus from that of the other muscles of the thigh situated more in front; this groove is known by a name which in this case is remarkably expressive--that of the 'line of poverty.'
If we examine the gluteal region as a whole by looking at the thigh from the side, we plainly see the graceful curve produced by the general convexity above indicated. We return to this point, in order to add that, in its lower part, this curve alters its character; that is to say, it is replaced by a slight concavity. This, which is designated under the name of _the fold of the buttock_, is situated close to the level of articulation of the leg with the thigh-bone.
First we recall that in man the anterior muscles of the thigh are: the triceps cruris, the tensor of the fascia lata, and the sartorius.
=Triceps Cruris= (Fig. 8, 36; Fig. 69, 31; Fig. 70, 41; Fig. 84, 2; Fig. 87, 3; Fig. 88, 3).--This muscle, which occupies the greater part of the space between the pelvis and the anterior aspect of the femur, consists of three parts: an external, or vastus externus; an internal, or vastus internus; and a median or long portion, or rectus femoris. This division accordingly recalls that which characterizes the human triceps cruris. Furthermore, as in the case of the latter, the vastus externus and the vastus internus take their origin from the shaft of the femur, while the long portion arises from the pelvis. The _vastus externus_ arises from the external lip of the linea aspera of the femur (or from the external border of the posterior surface of this bone in the ox and the horse, in which the linea aspera, considerably widened, especially in the latter, forms a surface), and from the external surface of the shaft of the femur. From this origin its fibres pass downwards and forwards, to be inserted into the tendon of the long portion of the muscle and into the patella.
In the dog and the cat the vastus externus is the most voluminous of the three portions which constitute the triceps muscle. It is covered by the fascia lata; but notwithstanding this, its presence is revealed by a prominence which occupies the external surface of the thigh, and surmounts, in the region of the knee, the more slightly developed one which is produced by the knee-cap.