THE ANTERIOR BRACHIAL REGION. 481 



of the humerus by the common tendon, from the internal lateral ligament of the 

 elbow-joint, and from the intermuscular septum common to it and the pre- 

 ceding muscles. The second head arises from the inner side of the coronoid 

 process of the ulna, above the ulnar origin of the Pronator radii teres (Fig. 200, 

 p. 255). The third head arises from the oblique line and from a portion of 

 the anterior border of the radius, extending to just below the insertion of 

 the Pronator radii teres. The fibres pass vertically downward, forming 

 a broad and thick muscle, which divides into four tendons about the 

 middle of the forearm ; as these tendons pass beneath the annular ligament 

 into the palm of the hand they are arranged in pairs, the anterior pair corre- 

 sponding to the middle and ring fingers, the posterior pair to the index and 

 little fingers. The tendons diverge from one another as they pass onward. 

 Opposite the base of the first phalanges each tendon divides into two slips, to 

 allow of the passage of the corresponding tendons of the Flexor profundus 

 digitorum : the two portions of the tendon then unite and form a grooved channel 

 for the reception of the accompanying deep flexor tendon. Finally they subdivide 

 a second time, to be inserted into the sides of the second phalanges about their 

 middle. After leaving the palm these tendons, accompanied bv the deep flexor 

 tendons, lie in osseo-aponeurotic canals formed by the fibrous sheath of the tendons 

 and the bones (Fig. 316). 



Relations. In the forearm, by its superficial surface, with the deep fascia and 

 all the preceding superficial muscles ; by its deep surface, with the Flexor profundus 

 digitorum. Flexor longus pollicis, the ulnar vessels and nerve, and the median 

 nerve. In the hand its tendons are in relation, in front, with the palmar fascia, 

 superficial palmar arch, and the branches of the median nerve ; behind, with the 

 tendons of the deep Flexor and the Lumbricales. 



Fibrous Sheath of the Flexor Tendons. The flexor tendons of the fingers as 

 they run along the phalanges are retained against the bones by a fibrous sheath, 

 forming osseo-aponeurotic canals. These sheaths are formed* by strong fibrous 

 bands which arch across the tendons and are attached on each side to the margins, 

 of the phalanges. Opposite the middle of the proximal and second phalanges the 

 sheath is very strong, and the fibres pass transversely; but opposite the joints it is 

 much thinner, and the fibres pass obliquely. Each sheath is lined by a synovia! 

 membrane, which is reflected on the contained tendon. 



Deep Layer. 



Flexor profundus digitorum. Flexor longus pollicis. 



Pronator quadratus. 



Dissection. Divide each of the supewcial muscles at its centre, and turn either end aside ; 

 the deep layer of muscles, together -^itb. the median nerve and ulnar vessels, will then be 

 exposed. jr 



The Flexor profundus digrcorum (perfbrans) (Fig. 307) is situated on the 

 ulnar side of the forearm, immediately beneath the superficial Flexors. It arises 

 from the upper three-fourths of the anterior and inner surfaces of the shaft of the 

 ulna, embracing the insertion of the Brachialis anticus above, and extending, 

 below, to within a short distance of the Pronator quadratus. It also arises from a 

 depression on the inner side of the coronoid process ; bv an aponeurosis from the 

 upper three-fourths of the posterior border of the ulna, in common with the Flexor 

 and Extensor carpi ulnaris : and from the ulnar half of the interosseous membrane. 

 The fibres form a fleshy belly of considerable size, which divides into four tendons : 

 these pass under the annular ligament beneath the tendons of the Flexor sublimis 

 digitorum. Opposite the first phalanges the tendons pass between the two slips of 

 the tendons of the Flexor sublimis digitorum, and are finally inserted into the 



- of the last phalanges. The tendon of the index finger is distinct ; the rest 

 are connected together by cellular tissue and tendinous slips as far as the palm of 

 the hand. The tendons of this and those of the Flexor sublimis digitorum, whilst 

 contained in the osseo-aponeurotic canals of the fingers, are invested in a synovial 



31 



