The palmar aponeurosis, fanning out from the distal border of the flexor retinaculum, is triangular in shape. From each of its two sides a fibrous septum dips downwards into the palm. That from the ulnar border is attached to the palmar border of the fifth metacarpal bone. Medial to it is the hypothenar space, not important surgically since it contains no long flexor tendons, but encloses only the hypothenar muscles. The remaining part of the palm is divided into two spaces by the septum that dips in from the radial border of the palmar aponeurosis to the palmar surface of the middle metacarpal bone. This septum lies obliquely and separates the thenar space on its radial side from the midpalmar space beneath the palmar aponeurosis. Normally the septum passes deeply between the flexor tendons of index and middle fingers; that is to say, the flexor tendons of the index finger overlie the thenar space. In some cases, however, the septum passes deeply between the index tendons and the first lumbrical; in these cases the first lumbrical overlies the thenar space whilst the flexor tendons to the index finger overlie the midpalmar space.
The midpalmar space is limited superficially by a thin fascia that lies deep to the common synovial sheath and the flexor tendons. The space is floored in by the interossei and metacarpals of the third and fourth spaces. Its sides are formed by the septa dipping in from the borders of the palmar aponeurosis. Infection in the space can readily break into the connective tissue canals surrounding the ulnar three lumbricals, so that for practical purposes the space may be considered to be continuous distally with these lumbrical canals. Proximally the space is closed by the firm attachment of the parietal layer of the common flexor synovial sheath to the walls of the carpal flexor tunnel.
The thenar space lies on the thumb side of the septum that joins the border of the palmar aponeurosis to the palmar border of the third metacarpal bone. It always contains the first lumbrical muscle, into whose canal infection can spread, as in the midpalmar space, and generally also it is overlaid by the flexor tendons to the index finger. In the palm, it is bounded posteriorly by adductor pollicis, but around the distal border of this muscle, in the web of the thumb, it is continuous with the slit-like space that lies between adductor pollicis and the first dorsal interosseous muscle. The thenar space is closed at the wrist, in a manner similar to the midpalmar space, by fusion of the parietal layer of the synovial sheaths with the walls of the carpal tunnel.