The fascia lata, descending from the pos terior surface of the thigh, forms the strong aponeurosis which closes in the popliteal space behind ; stretched across this region, it is connected on either side with the condyles of the femur and the tendons of the extensor muscles of the leg, and continued around the joint ; it is especially fixed to the outer lip of the linea aspera by a septal process of the fascia lata which dips between the vastus ex ternus and biceps muscles, and on the inner side receives fibres from the tendons of the muscles which pass behind the inner condyle; it approximates the lateral boundaries of the space, gives to it a greater depth, and protects the vessels and nerves by bearing offfrorn them any undue pressure. By its unyielding and dense structure, aneurismal or other swellings are delayed in their approach to the surface, and for the same reason abscesses are prone to burrow and require an early and free opening. This aponeurosis presents numerous trans verse fibres, is perforated sometimes by the saphena minor vein, and is adherent by fibrous slips with the subcutaneous areolar tissue ; it is continuous below with the aponeurosis of the leg.' The muscles forming the boundaries of the popliteal space will be considered only so far as they relate to it ; their more detailed descrip tion will be found in the articles MUSCLES OF THE LEG AND THIGH. The semitendinosus muscle, after separating from the biceps, termi nates in a long, slender tendon, which descends inwards, lying upon the surface of the semi membranosus ; then crosses the inner head of the gastrocnemius, and placed between it and the tendon of the semimembranosus, winds round the inner condyle to pass to its insertion. The semimembranosus in descending at first crosses obliquely the popliteal artery, continues membranous and fleshy to the condyle of the femur, and is of sufficient breadth, at this its lower part, to extend beyond either side of the tendon of the former muscle, thus contracting the lateral dimensions of the bottom of this space by encroaching within it. The long head of the biceps, leaving the former muscles, de scends obliquely to cross the outer origin of the gastrocnemius by its strong tendon, which is subsequently implanted into the head of the fibula ; it receives, as it descends, the thick fleshy mass of its shorter portion, which assists the semimembranosus in narrowing the bottom of the popliteal space, shutting it in also exter nally and above, by its attachment to the linea aspera as low as the outer condyle. The supe rior angle of this space is formed at the point of divergence of these hamstring muscles, and the lateral angles are occasioned by their crossing the heads of the gastrocnemius ; this latter muscle is attached to either condyle of the femur by its two heads, the internal being the longer and larger ; they converge to unite in the median line a little below the knee-joint : these origins have each a bursa interposed be tween them and the condyle ; the little fleshy belly of the plantaris muscle accompanies the outer head and lies beneath it ; by their union the inferior angle of the popliteal region is pro duced. At the bottom of the space we meet with, first, the posterior, flat, triangular surface of the femur, and, secondly, the back part of the knee-joint strengthened by its posterior ment (the ligament of Winslow). This struc ture is derived from the tendon of the semi membranosus ; it insinuates itself beneath the inner head of the gastrocnemius, and, forming a flat and dense tendinous aponeurosis, ex tends across the back of the joint to the external con dyle,adheringto the synovial membrane ; there are several small openings in it, produced by a separation of its fibres, for the passage of vessels to the interior of the ai ticulation : —lastly, the popliteus muscle, which is flat, triangular, and situate behind and below the joint, begins, by a round tendon, from the outer condyle and spreads out by muscular fibres upon the poste rior surface of the tibia to be inserted into its oblique ridge.
On removing the fascia, two large nerves are seen to traverse the popliteal space, and are in differently called the internal popliteal or tibial, and the external popliteal or peroneal nerves ; they are the terminal branches of the sciatic, winch nerve generally bifurcates at the upper angle of this region : the point of division, how ever, is very variable, sometimes occurring even within the cavity of the pelvis, in which case, as the two nerves emerge, they are usually sepa rated by a slip of the pyriforrnis muscle ; com monly a very trifling dissection will affect their separation some distance up the thigh. The internal popliteal or tibial nerve is the larger of the two, and appears to be the continuation of the sciatic ; it takes a nearly perpendicular course through the popliteal space in the middle line, and will be found at first to lie almost im mediately beneath the fascia, a small quantity of fat intervening ; it dips more deeply into the space as it descends, passes between the heads of the gastrocnemius and over the popliteus muscle, and, insinuating itself beneath the ten dinous arch of the soleus, courses down the back of the leg under the name of posterior tibial. Owing to the oblique direction which the popliteal artery follows, this nerve alters its relation to it at different parts of its course ; until they reach the bend of the knee, the nerve is a little distance to the outer side of the artery, but superficial or posterior, and separated from it by a thick layer of adipose tissue : at the joint, the nerve is still posterior to,but in closer relation with it, and subsequently upon the popliteus muscle crosses the artery to gain its inner side. About the centre of the popliteal space, the tibial nerve sends off a small branch called the communicans tibialis, which descends superficially between the heads of the gastro cnemius, and is afterwards con cealed in a groove formed by their union ; it perforates, at a va riable point, the aponeurosis of the leg, and, de scending towards the outer malleolus, is joined a little above it by the communicans peronei, a branch of the peroneal nerve ; thus reinforced, it is increased in size, and, accompanied by the posterior saphena vein, winds behind the outer ankle to be continued along the outer side of the foot. To return to the internal popliteal nerve, which sends off, while crossing the back of the joint, four or five other branches for distribu tion to the gastrocnemii and plantaris muscles, and also furnishes some articular twigs ; these are all accompanied by corresponding branches of the popliteal artery, and from their situation are liable to be compressed by an aneurismal tumour. The external popliteal or peroneal nerve descends along the inner side of the bi ceps muscle, by which it is giiided to the head of the fibula, and winds round the neck of that bone beneath the peroneus longus muscle to divide into its terminal branches ; in the ham it gives off the small branch called the communicans peronei. This will be seen to de scend over the outer head of the gastrocnemius muscle and beneath the fascia, and, piercing the aponeurosis of the leg at a very variable distance above the outer angle, joins the com municans tibialis ; it presents frequent varieties both with regard to its size and the point of junction with the last-named nerve : occasion ally the union occurs in the popliteal space. To reach the popliteal vessels, a quantity of fat which fills up this space must be dissected out : it is very abundant, and surrounds and supports the popliteal artery.