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.
The vastus internus, situated on the inner surface of the thigh, takes its origin from the corresponding surface of the femur, and proceeds towards the patella.
The rectus femoris arises from the iliac bone, above the cotyloid cavity; its fleshy body, which is fusiform, and situated in front of and between the two vasti muscles, is directed towards the patella, into which it is inserted by a tendon, which receives the other two portions.
It is covered in front by the tensor of the fascia lata, and contributes with the vastus externus to form the upper prominence of the knee.
The ligamentous fibres, which, as in man, unite the knee-cap to the tibia, transmit to this latter the action determined by the contraction of the triceps. This muscle is an extensor of the leg. Furthermore, the rectus femoris, or long portion, acts as a flexor of the thigh.
Tensor Fascia Lata ([Fig. 68], 34, 36; [Fig. 69], 30, 31; Fig. 70, 40).—This muscle, generally larger in quadrupeds than in man, is flat and triangular, and occupies the superior and anterior part of the thigh.
It arises from the anterior iliac spine (inferior in carnivora, external in the ox and the horse); it is prolonged downwards by an aponeurosis (fascia lata) which occupies the external aspect of the thigh, proceeds to be inserted into the patella and blend with the aponeurosis of the biceps muscle.
It covers the rectus and vastus externus portions of the triceps cruris; it is also in relation with the gluteal muscles.
The tensor of the fascia lata flexes the thigh, and serves to raise the lower limb as a whole.
Sartorius ([Fig. 68], 35; [Fig. 87], 4, 5; [Fig. 88], 5).—This muscle, long and flattened, is called by veterinarians the long adductor of the leg.