The three web spaces of the palm lie between the four slips of attachment of the palmar aponeurosis. From the skin edge they may be said to extend proximally as far as the metacarpophalangeal joints, a distance of about 4 cm. Between the palmar and dorsal layers of the skin lie the superficial and deep transverse ligaments of the palm, the digital vessels and nerves, and the tendons of the interossei and lumbricals on their way to the extensor expansions. The web is filled in with a packing of loose fibro-fatty tissue.
The superficial transverse metacarpal ligament lies beneath the palmar skin across the free margins of the webs. Its fibers lie transversely in a flat band, best developed on the radial side and thinning out over the more mobile little finger. Its two ends, and the parts that lie across the fibrous flexor sheaths, are continued along the sides of the fingers, tethering the skin to the phalanges. The ligament supports the fold of skin at the web (it is well developed in quadrupeds whose forefeet are webbed). The digital vessels and nerves lie immediately deep to the ligament, a point to be remembered in making web incisions for palmar space infections. Here the nerves lie on the palmar side of the arteries. The lumbrical tendon lies beneath the vessels, passing distally along the lumbrical canal into the radial side of the dorsal expansion of its own finger.
The deep transverse metacarpal ligament joins the palmar ligaments of the metacarpophalangeal joints. It lies 3 cm proximal to the superficial transverse ligament.
The interosseous tendons lie on the dorsal side of the deep transverse ligament; the lumbrical tendons are on the palmar side.
The web of the thumb lacks both superficial and deep transverse ligaments, a factor contributing to the mobility of the thumb. The deep fascia passes across from palmar to dorsal surfaces of the web and beneath it lie the transverse head of adductor pollicis and the first dorsal interosseous muscle. From the slit-like space between them emerge the radialis indicis and princeps pollicis arteries. Each hugs its own digit and the central part of the web can be incised without risk to either vessel. The thenar space lies beneath the deep fascia on the palmar surface of adductor pollicis; the first lumbrical passes through the space to the radial side of the index finger.