MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 117 



for the adjacent sides of the index and middle finger. The third is 

 innervated by both the median and the dee]) branch of the idnar 

 nerve; the median supphes it through its ramus to the ulnar side of 

 the middle finger and the ulnar by the continuation of its ramus to 

 the adductor poUicis and contrahens II. The fourth lumbrical receives 

 its innervation from the deep ulnar nerve by the continuation of its 

 ramus to contrahens V. 



Function: They are all flexors of the metacarpophalangeal joints. 



DEEP SERIES, SUPERFICIAL LAYER 



The layer of contrahentes is represented by three muscles, namely > 

 to the first, second, and fifth digits. There is no contrahens to the fourth 

 digit. 



M. adductor pollicis (fig. 34): This is a broad, thin, and triangular 

 muscle whose origin corresponds to approximately the midUne of 

 metacarpale IV. It is superficial to the contrahens indicis and arises 

 by muscular fibers from the palmar base of metacarpals II and III, 

 capitate, and the ligamentous bindings of these bones. Fibers of the 

 muscle come also, through a fine resistant aponeurotic lamina, from 

 a sagittal septum attached to ths palmar side of the fourth metacarpal 

 shaft. The carpal bundles pass distolaterally, while those of metacarpal 

 origin follow an increasingly horizontal direction. The insertion is by 

 fleshy fascicles on the ulnar sesamoid of the first digit and the same 

 side of the base of the first polhcal phalanx. No separate obUque and 

 transverse heads are to be found, but the radial border of the muscle 

 is considerably thicker than the rest. Sirena (1871) also found the 

 muscle not divided into obUque and transverse heads. He assigns the 

 insertion to the whole length of the inner margin of the first pollical 

 phalanx. 



Nerve supply: By a branch of the deep ulnar nerve. 



Function: Adductor of the poUex. 



M. contrahens indicis: This is similar to but smaller than that for 

 the first digit, under cover of which it lies (fig. 34). Its origin is from the 

 palmar aspect of the third metacarpal base and capitate; more distally 

 its fibers arise from the sagittal septum to the fourth metacarpal by 

 means of the same aponeurotic lamina already described with the 

 adductor pollicis. They all converge to the space between the heads 

 of metacarpals II and III, which is crossed on the dorsal side of the 

 corresponding deep transverse metacarpal hgament. The muscle ends 

 on the ulnar side of the base of the proximal phalanx of digit II. 

 Sirena (1871) calls this muscle m. adductor indicis obliquus. 



Nerve supply: The same branch of the ulnar nerve which also 

 innervates the adductor poUicis and the third lumbrical. 



Function: Adducts the second digit. 



