1208 SURFACE AND SURGICAL ANATOMY. 
the lower third of this surface is an oblique prominence caused by the extensors 
of the metacarpal bone and first phalanx of the thumb. 
The flexor sheaths of the palm and of the digits are of surgical importance in 
consequence of their lability to suppurative inflammation. The common flexor sheath 
begins 14 in. above the annular ligament, under which it extends to a little 
below the middle of the palm. The digital flexor sheaths extend from the bases of 
the terminal phalanges to the level of the distal transverse crease of the palm, 
opposite the necks of the metacarpal bones, with the exception of the sheath of the 
little finger, which is continuous with the common flexor sheath of the palm. The 
sheath of the flexor longus pollicis extends from the base of the terminal phalanx 
upwards to a point about 1 in. above the annular ligament; it frequently com- 
municates with the common flexor sheath. From this anatomical arrangement 
it follows that suppuration in the sheaths of the little finger and thumb is 
specially liable to spread upwards into the palm, and thence underneath the 
annular ligament into the forearm. 
The pulsations of the radial artery can readily be felt in the lower third of the 
forearm, midway between the outer border of the radius and the tendon of the 
flexor carpi radialis. The course of the vessel is indicated upon the surface by 
a line extending from the bifurcation of the brachial (} in. below the middle of 
the bend of the elbow) to the tubercle of the scaphoid, around which, and below 
the tip of the styloid process, the artery winds to the back of the radial side of the 
wrist ; in the latter situation the vessel, after passing beneath the extensor tendons 
of the thumb, dips ito the paln through the proximal extremity of the first inter- 
osseous space. Incisions for opening or resecting the wrist are planned so as to 
avoid the vessel. 
The upper third of the ulnar artery is deeply placed, and takes a curved course 
from the bifurcation of the brachial towards the imner part of the anterior surface 
of the forearm; the lower two-thirds of the vessel correspond to the lower two- 
thirds of a line drawn from the front of the internal condyle to the inner border of 
the pisiform bone. The course of the ulnar nerve corresponds to the whole of the 
above line. 
The median nerve in the forearm may be mapped out by a line extending from 
a point midway between the centre of the bend of the elbow and the internal epi- 
condyle, to a point midway between the styloid processes; in the lower third of 
the forearm the line follows the inner border of the tendon of the flexor carpi 
radialis. To evacuate pus spreading deeply up the front of the forearm, the 
incisions should be made on either side of the line corresponding to the median 
nerve. The radial nerve winds to the back of the forearm round the outer border 
of the radius beneath the tendon of the supinator longus, at the junction of the 
middle and lower thirds of the forearm. The summit, or most distal part of the 
superficial palmar arch, corresponds to the mid-point of a line extending from the 
middle of the lowest transverse crease of the wrist to the root of the muddle 
finger; a line drawn from the outer border of the pisiform bone across the hook 
of the unciform, and thence in a curved direction downwards and outwards to this 
point, corresponds to the main or proximal part of the arch; the first and fourth 
digital branches overlie the fifth and third metacarpal bones respectively, while 
the second and third overlie the fourth and third interspaces respectively. The 
deep arch lies almost transversely, midway between the lower border of the anterior 
annular ligament and the superficial arch. The radialis indicis corresponds to the 
radial border of the index- finger. 
The ulnar nerve and the commencement of its two divisions lie immediately to 
the inner side of the superficial palmar arch, so that the pisifori and the hook of 
the unciform are the guides to the nerve. The median nerve emerges from beneath 
the annular ligament opposite the inner edge of the thenar eminence, while the 
digital branches to the thumh follow its lower margin. Incisions for the removal 
of foreign bodies may therefore be made into the thenar with ereater freedom 
than into the hypo-thenar eminence. 
Incisions to evacuate deep-seated pus in the palm may be made in one or more of 
the following situations: (1) over the lower two-thirds of the second metacarpal bone ; (2) 
