Before beginning the study of its position in quadrupeds, it is necessary to remember that in man, where the thigh has a form almost conical, the sartorius commences on the anterior face of this latter, and is directed downwards and inwards to reach the internal surface of the knee.
But now let us suppose the thigh flattened from without inwards; there will evidently result from this a change in situation with regard to the muscle in question. In fact, when this supposition is admitted, it is easy to imagine that in a great part of the extent in which the sartorius is normally anterior it will become internal. This is why, these conditions being realized in quadrupeds, we shall find that, in some of them, the sartorius is situated on the aspect of the thigh which is turned to the side of the trunk.
In the dog and the cat it arises from the anterior iliac spine, and from the half of the border of the bone situated immediately below it; but the fibres from this second origin being hidden by the tensor of the fascia lata, on the inner side of which they are situated, viewing the external surface of the thigh, the muscle seems to arise from the iliac spine only.
The sartorius in these animals is divided into two parts, which, in general, are placed in contact. One of these fasciculi is anterior; the other is situated further back. The first is visible on the anterior border of the thigh, in front of the tensor of the fascia lata, but below it inclines inwards; in its superior part also, a small extent of the internal surface is occupied by it. The second, which, as we have said, is situated further back, belongs wholly to the inner surface of the thigh; it is this portion which arises from the inferior border of the ilium (this is the homologue of the anterior border of the human iliac bone).
The two fasciculi then pass towards the knee, being in relation with the rectus and the vastus internus of the triceps. The anterior fasciculus is inserted into the patella. The posterior unites with the tendons of the gracilis (see below) and semi-tendinosus, and then proceeds to be inserted into the superior part of the internal surface of the tibia.
On account of their different insertions these two parts receive the names of the patellar sartorius and tibial sartorius respectively.
In the ox and the horse the sartorius is still more definitely situated on the internal surface of the thigh. Consisting of a single fasciculus, representing the tibial sartorius of the cat and the dog, it arises in the abdominal cavity from the fascia covering the iliac muscle, then passes under the crural arch, and terminates, by an aponeurosis which blends with that of the gracilis, on the inner fibres of the patellar ligament. In short, the sartorius is of interest to us in the carnivora only, and especially on account of its anterior or patellar fasciculus.
It is an adductor of the leg and a flexor of the thigh.
Muscles of the Internal Region
The ilio-psoas pectineus and the adductors which we study in man, in connection with the internal aspect of the thigh, offer little of interest from the point of view of external form in quadrupeds; it is for this reason that we will disregard them.