THE MUSCLES OF THE UPPER LIMB. 



opens. The tendons of the ring and little fingers are most extensively surrounded by the 

 synovial membrane, and those of the index and middle fingers are but slightly in relation 

 with it. A sagittal septum, placed behind the median nerve, separates this sac from that 

 investing the tendon of the flexor longus pollicis. In exceptional cases an intermediate sac is 

 formed on the deep flexor tendon of the index finger ; or there may be separate sheaths on the 

 superficial flexor tendons of the index and middle fingers. In the infant, the digital synovial 

 sheaths, including that of the thumb, are at first distinct from the sacs beneath the annular 

 ligament. (A. v. Rosthorn, Langenbeck's Arch. f. klimsche Chirurgie, xxxiv, 1887.) 



The sheaths of the flexor tendons, by which they are bound down fco the fingers, 

 are formed opposite the shafts of the first and second phalanges by strong tendinous- 

 looking bands of transverse fibres, vaginal ligaments, attached to the rough margins 

 of the palmar surfaces of the phalanges. Opposite the joints, flexion is allowed by 

 the substitution for those bands of a thin membrane, strengthened by oblique decus- 

 sating fibres. The tendinous sheath has a synovial lining, which gives a separate 

 investment to each tendon. 



The synovial membrane forms small folds (vincula accessoria tendinum) between the 

 tendons and the bones. There are two sets of these ; the one, ligamenta brevia, broad and 

 membranous, passing between the tendons near their insertion and the lower part of the 

 phalanx immediately above ; the other, ligamenta longa, slender and less constant bands, 

 joining the tendons at a higher level. Contained in the ligamentum breve of the deep flexor 

 is a small band of yellow elastic tissue (vinculum subflav am), which stretches from the tendon 

 to the head of the second phalanx, and may assist in drawing down the tendon after flexion 

 of the fingers (J. Marshall, Brit, and For. Med. Chir. Rev., 1853). 



The flexor 'longus pollicis muscle, placed side by side with the flexor pro- 

 fiindus digitorum, arises from the anterior surface of the radius, extending from the 

 oblique line to the edge of the pronator quadratus, and from the adjacent part of the 

 interosseous membrane ; and in the majority of instances it receives also a distinct 

 fleshy and tendinous slip, springing in common with the flexor sublimis digitorum 

 from the inner condyle or the coronoid process. The muscle ends in a tendon which 

 passes behind the annular ligament of the wrist close to the trapezium, turns out- 

 wards between the outer head of the flexor brevis and the adductor obliquus pollicis, 

 and lying in the groove between the sesamoid bones, finally enters a canal similar to 

 those of the other flexor tendons, to be inserted into the base of the second phalanx 

 of the thumb. 



As the tendon of the muscle passes under the annular ligament it is surrounded by a 

 synovial sac, which is continued below into the digital sheath, and occasionally communicates 

 above with the bursa of the common flexor tendons. 



Varieties. The flexor longus pollicis is sometimes connected by a slip with the flexor 

 sublimis or profundus, or the pronator teres. A tendon of insertion into the index finger has 

 been observed, as also a slip to the first lumbricalis. 



The complete separation of the flexor longus pollicis is characteristic of man. In other 

 Primates it is variously united with the flexor profundus digitorum. 



The pronator quadratus, placed close to the bones behind the last two muscles, 

 arises from the pronator ridge and the adjoining inner part of the anterior surface 

 of the ulna in its lower fourth ; its fibres cross the lower part of the forearm, some 

 transversely and others obliquely, and they are inserted for a slightly shorter distance 

 into the fore part and inner side of the radius. 



Varieties. The pronator quadratus is subject to varieties, chiefly as follows : 1, it may 

 be entirely absent, but this is rare ; 2, it is subdivided into two layers, or occasionally into 

 three ; 3, it extends farther upwards on the bones of the forearm than usual ; 4, it is pro- 

 longed downwards on the carpus, in some cases as a radio-carpal, and in others as an ulno- 

 carpal muscle. 



The radio-carpeus or fexor carpi radialis brevis is an additional small muscle not 

 unfrequently seen, arising from the radius, usually from the anterior border and surface 

 above the pronator quadratus, and very variably inserted below, into the annular ligament, 



