6io HUMAN ANATOMY. 



{bb) The Muscles of the Second Layer. 



In the lower vertebrates the second layer also arises from the palmar aponeu- 

 rosis, but from its deeper layers. These, as has been stated (page 597), differentiate 

 into the palmar portions of the tendons of the flexor profundus digitorum, and in 

 the mammalia the muscles retain their primary origin and arise from those tendons 

 forming the lumbrical muscles. 



I. LUMBRICALES (Fig. 578). 



Attachments. — The lumbricals are four slender, band-like muscles, situated 

 in the palm of the hand. Counting from the radial side of the hand, the Jirst and 

 second lumbricals arise from the radial side of the flexor profundus tendons to the 

 index and middle fingers respectively, while the third one arises from the adjacent 

 sides of the tendons to the middle and ring fingers, and the fourth from those of the 

 tendons to the ring and little fingers. The muscles pass distally into slender tendons 

 which are continued to the radial side of the first phalanges of the second, third, 

 fourth, and fifth lingers, and are inserted into the membranous expansions of the 

 tendons of the extensor communis digitorum to those fingers. 



Nerve-Supply. — The first and second lumbricals are supplied by the median 

 nerve from the sixth and seventh cervical nerves ; the third and fourth by the deep 

 division of the ulnar nerve from the eighth cervical and first thoracic nerves. 



Action. — To flex the first phalanges of the second, third, fourth, and fifth 

 fingers. At the same time, by their traction upon the extensor tendons, they will 

 tend to keep the second and third phalanges extended. 



Variations. — Variations in the arrangement of the lumbricals, and especially of the third 

 and fourth, are not uncommon. The tendon of each of these muscles may bifurcate and be in- 

 serted into the adjacent sides of the third and fourth or fourth and fifth fingers, and more 

 rarely the sole insertions may be into the ulnar sides of the first phalanges of the middle 

 and ring fingers. The third lumbrical is frequently supplied wholly or in part from the median 

 nerve. 



{cc) The Muscle of the Third Layer.. 



In the lower vertebrates the third layer consists of muscles which arise from the 

 carpal and metacarpal bones and pass to each of the digits. In) the mammalia they 

 become greatly reduced in number, frequently persisting, however, in connection 

 with the thumb, index, and little fingers, but in man they are represented only by 

 an adductor pollicis. 



I. Adductor Pollicis (Figs. 578, 588). 



Attachments. — The adductor pollicis is a flat triangular muscle which rests 

 upon the metacarpal bones and the interosseous muscles. It may be regarded as 

 consisting of two portions. The portio obliqua (often described as a distinct muscle, 

 the adductor obliquus pollicis^ arises from the trapezium, trapezoid, and os magnum 

 and from the bases of the second and third metacarpals. Its fibres are directed dis- 

 tally and radially, and are inserted by a tendon, in which a sesamoid bone is usually 

 developed, into the ulnar side of the base of the first phalanx of the thumb. It 

 also sends off a slip which passes beneath the tendon of the flexor longus pollicis to 

 be inserted into the radial side of the base of the first phalanx of the thumb along 

 with the flexor brevis pollicis. 



The portio trayisversa (often described as the adductor transversus pollicis') 

 arises from the lower two-thirds of the volar surface of the third metacarpal, and its 

 fibres pass almost directly radially to be inserted into the ulnar side of the base of 

 the first phalanx of the thumb. 



Nerve-Supply. — By the deep division of the ulnar nerve from the" eighth 

 cervical and first thoracic nerves. 



Action. — To adduct the thumb. 



Relations. — The adductor pollicis is covered by the tendons of the flexor 

 profundus digitorum for the second and third fingers and by the first and second 

 lumbricals. It conceals the interosseous muscles of the two radial intermetacarpal 

 intervals and also the radial artery and the arteria princeps pollicis. The deep 

 palmar arch passes between the two portions of the muscle, near their origins. 



