410 



THE MUSCULATURE 



others from three-fourths of the shaft. The fibre-bundles of each muscle converge in a penni- 

 form manner upon a tendon which is inserted into the aponeurosis of the digital extensor tendon 

 and the base of the first phalanx on the middle finger side of the corresponding digit (see fig. 373). 

 The first volar interosseous is often described as a division of the flexor poUicis brevis or of the 

 adductor polhcis. 



The interossei dorsales arise from the adjacent sides of the metacarpal bones in each inter- 

 space. On the sides nearest the middle finger they cover three-fourths of the bone, on the 

 opposite sides much less. The fibre-bundles converge in a bipenniform manner upon a tendon 

 which begins high in the muscle and is inserted into the aponeurosis of the extensor muscles 

 and the base of the first phalanx on each side of the middle finger, on the thumb side of the 

 index finger, and the ulnar side of the ring finger. The interosseous muscle in the first interspace 

 is thick and strong and forms with the adductor polUcis the fleshy web between the base of the 

 thumb and the palm. 



Nerve-supply. — By branches of the deep palmar division of the ulnar nerve. As a rule, a 

 branch to each volar interosseous enters the proximal third of the muscle. To each dorsal inter- 

 osseous a branch is given which enters between the two heads. These branches may be variously 

 combined before entering the interosseous muscles. The nerve fibres arise from the eighth 

 cervical and first thoracic nerves. 



Fig. 379. — The Dorsal Interossei. 



Action, — To move the fingers toward the radial and ulnar sides, to flex the first phalanx 

 and extend the second and third. The volar interossei move the fingers toward the median 

 axis, the dorsal from it. 



Relations. — The volar interossei lie volarward from the dorsal interossei. The two sets 

 of muscles are bound in place by the dorsal and volar metacarpal fascijB. The tendons pass out 

 on the dorsal side of the transverse capitular ligament and are closely applied to the metacarpo- 

 phalangeal joints. The muscles of the first two interspaces lie immediately dorsal to the adduc- 

 tor of the thumb; the others dorsal to the flexor tendons. 



Variations. — The tendon shp from an interosseous muscle to the base of the first phalanx 

 of a digit may be missing. This is more frequent in case of the volar than in that of the dorsal 

 interossei, and in the medial than the lateral muscles. Either a volar or a dorsal interosseous 

 muscle may be double or missing. Rarely the insertions of the interosseous muscles character- 

 istic of the foot (see p. 499) may be found in the hand. 



III. SPINAL MUSCULATURE 



(Figs. 380, 381, 382, 383) 



The spinal (vertebral) column is of special interest as the segmented longitudi- 

 nal axial support of the body which has given rise to the term "vertebrates" as 

 applied to the class of animals of wiiich man is the highest form. The segmenta- 

 tion in fishes permits the lateral movements of the body which are their chief 

 means of propulsion. In the land vertebrates, with the exception of snakes, the 

 limbs are developed as the chief organs of propulsion but flexibility of the column 



