148. =Furrow in palm.=--The only furrow in the palm useful as a surgical landmark is that which runs transversely across its lower third, and is well seen when the fingers are slightly bent. This transverse furrow corresponds pretty nearly with the metacarpal joints of the fingers, with the upper limit of the synovial sheaths of the flexor tendons of the fingers (that of the little finger excepted (143)); also with the splitting of the palmar fascia into its four slips. The transverse metacarpal ligament lies in the same line with it. Again, a little below this furrow, the digital arteries bifurcate to run along the opposite sides of the fingers. 149. =Interdigital folds.=--By pressing upon the interdigital folds of skin, we can feel the transverse ligament of the fingers, which prevents their too wide separation. The skin of these folds is much thinner on the dorsal than the palmar aspect; hence deep-seated abscesses in the palm very frequently burst on the back of the hand. 150. =Digital furrows.=--Concerning the transverse furrows on the palmar surface of the fingers, notice that the first furrows, close to the palm, do not correspond with the metacarpal joints. The second and third furrows do correspond with their respective joints. The slight depression observable between the ball of the thumb and that of the little finger corresponds with the middle of the anterior annular ligament. 151. =Palmar arterial arches.=--In opening abscesses in the palm, it is important to bear in mind the position of the large arterial arches which lie beneath the palmar fascia. The line of the superficial palmar arch crosses the palm about the junction of the upper with the lower two-thirds--that is, in the line of the thumb separated widely from the fingers. From this, the digital arteries run straight between the shafts of the metacarpal bones towards the clefts of the fingers. Incisions, therefore, to let out pus beneath the palmar fascia may safely be made in the lower two-thirds of the palm, provided they run in the direction of the middle line of the fingers. The deep palmar arch lies half an inch nearer the wrist than the superficial. 152. =Digital arteries.=--As the digital arteries run along the sides of the fingers, the incision to open a thecal abscess should be made strictly in the middle line. It should be made not over but between the joints, since the sheath is strongest and thickest over the shafts of the phalanges, and therefore more likely to produce strangulation of the enclosed tendons.