THE POPLITEAL ARTERY is simply a conti nuation of the femoral, and is so named immedi ately after the latter vessel has passed through the elliptical aperture of the adductor muscles ; this opening is bounded above by the united tendons of the adductor longus and adductor magnus muscles ; inferiorly, by the union of the vastus internus tendon with that from the ad ductor magnus which descends to the inner condyle ; externally, bythe tendon of the vastus internus, and internally, by that of the adductor magnus. Passing through this tendinous aper ture, the artery is at first situated on the inner side of the femur at the junction of its middle and inferior thirds, and descends obliquely outwards and from before, backwards through the popliteal space, to the lower border of the popliteus imuscle, where it terminates, after having gradually diminished somewhat in size, by dividing into the anterior and posterior tibial arteries. When viewed with regard to the vertical axis of the popliteal region, the artery certainly takes an oblique course out wards ; but in reference to the mesial and perpendicular line of the body, this obliquity is more apparent than real, and depends upon the direction inwards which the shaft of the thigh bone follows ; and this appears evident by the artery passing vertically and midway between the condyles of the femur. Its course from before backwards is very decided until it has attained the superior border of the popli tens muscle; but as the lower portion of the popliteus is on a plane a little anterior to the upper, and as the artery is applied upon its posterior surface the course will be changed for a direction forwards, so that the artery describes a slight curve, convex backwards, and the concavity corresponding with the back of the knee-joint. When the leg is flexed upon the thigh, the popliteal artery follows the bend of the articulation, and is curved forwards without lateral tortuosity, the curve agreeing with the angle of flexion ; this alternate straightening and bending of the artery during the movements of the leg has been assigned as a reason for its being so frequently the seat of aneurism ; on the other hand, it has been stated that forced extension of the leg, carried even to rupture of the liga ments of the joint, may be made without in jury to the artery. The popliteal artery is closely related to its accompanying vein ; as they are entering the space, the vein lies to the outer side of the artery, and superficial or posterior to it, and changes its relation near the joint only to become still more directly posterior : they are enveloped in a common sheath, which is continued from the femoral region (see FEMORAL ARTERY), and by which they are intimately connected with each other. The artery is at first deeply seated in the popliteal region, and guided into it by the inferior boundary of the elliptical tendinous opening ; it then descends obliquely upon the flat triangular surface of the femur to the knee-joint, resting in its course upon a cushion of fat which is interposed between it and the bone, and thicker below than above, so as to well support the artery as it inclines back wards from the femur to reach the posterior aspect of the joint. For some distance from its commencement it is concealed beneath the semimembranosus muscle, the thick fleshy belly of which obliquely crosses it behind ; emerging from under cover of this muscle, the artery continues its course to the condyles of the femur, between the biceps on the outer side, and semimembranosus and semitendinosus on the inner ; a considerable quantity of fat sepa rates it from, posteriorly, the aponeurotic fascia, closing in the space behind, and from the skin. As the internal popliteal or tibial
nerve descends vertically in the axis of this region, it must necessarily lie to the outer side of the artery in this part of its course ; and as the nerve is found almost immediately beneath the fascia, it is therefore superficial or posterior to the artery, from which it is sepa rated by more or less fat. While thus buried in fat, three or four lymphatic glands are closely related to the artery, often indeed sur rounding it, one to either side, another super ficial, and a fourth occasionally found between it and the femur. Should any of these glands become enlarged, the impulse such swelling would receive from the artery might lead to its being mistaken for aneurism. We next find the popliteal artery crossing the bend of the knee-joint, and resting upon its posterior ligament ; it descends between the condyles of the femur and the two heads of the gas trocneraius to the upper border of the popli-. teus muscle : the little fleshy belly of the plantaris is also related to its outer side. In this stage the accompanying vein is more directly behind it, and the tibial nerve, coining into closer relation with the artery, from which it is separated by the vein, is also pos terior or superficial to it, with a tendency to cross to its inner side. At this part of its course the nerve usually sends off, first, the communicans tibialis, and then its branches to the heads of the gastrocnemius, so that the relation which the nerve and its branches have to the artery at this point will readily account for the pain or numbness generally attendant on aneurismal tumours in this region; so, also, for cedematous swelling of the leg under the same.circumstances, we have only to refer to the relative anatomy of the vein and artery for its explanation ; posteriorly, the artery is sepa rated from the fascia and integument by more or less fat, and is still a considerable distance from the surface ; for the tendons of the ham string muscles, and the condyles of the femur with the heads of the gastrocnemius, so bear off from the artery the skin and fascia as to leave it in a deep and narrow hole, resting upon the posterior ligament of the joint, and con cealed behind by, first, the vein, and then the tibial nerve. Of course any operation upon the artery while thus situated would be impracti cable. La.stly, the artery gains the posterior surface of the popliteus muscle, upon which it descends to terminate by dividing into the anterior and posterior tibial vessels ; this divi sion occurs at the lower border of the muscle, and opposite the interval between the tibia and fibula. The artery is deeply concealed between the heads of the gastrocnemius as they ap proach each other to unite; the tibial nerve crosses to gain its inner side, and the vein, which often receives the tibio-peroneal vein while upon the popliteus, is still posterior to the artery.