276 



MORPHOLOGY OF THE LIMB-MUSCLES. 



liinb they are confined to the foot, there is reason for believing 1 that the latter condition is the 

 primary one (H. St. John Brooks, " The Morphology of the Extensor Muscles." Proc. Anat. 

 Soc., 1888, and " Studies from the Museum of Zoology," University College, Dundee, 1889). 



The short muscles of the digits may be derived from a typical arrangement consisting of 

 1, a superficial layer of four muscles, primarily adductors ; 2, an intermediate layer of bicipital 

 short flexors, one for each digit ; and 3, a deep, or dorsal, layer of abductors, six in number. 

 The deep branch of the ulnar (or external plantar) nerve crosses between the first and second 

 layers. In "the hand, the outermost muscle of the first layer is largely developed, forming 

 the adductors of the thumb, the innermost muscle gives rise to the flexor brevis and the 

 larger part of the opponens minimi digiti, and the two central muscles are suppressed. The 



DVI TJV 



DUI DTI DT 



w w w If 



NERVE: nv 



ULNAR NERVE 



ADD. POLL. 



Fig. 265. DIAGRAMS SHOWING 



THE ARRANGEMENT OF THE 

 SHORT DIGITAL MUSCLES IN 

 TRANSVERSE SECTIONS OF THE 

 METACARPAL OR METATARSAL 

 REGIONS. A, TYPICAL AR- 

 RANGEMENT ; B, MUSCLES OF 



HAND J C, MUSCLES OF FOOT. 



(Based on Cunningham.) 



(GK D. T.) 



I V, first to fifth inetacarpal 

 or metatarsal bones ; D I D VI, 

 the dorsal layer of abductors ; 

 F I F V, the intermediate layer 

 of short flexors ; P I P IV, the 

 plantar layer of adductors. In 

 B and C, the absent elements are 

 indicated by dotted outlines. 



two heads of the short flexor 

 of the thumb are retained, the 

 opponens pollicis being a seg- 

 mentation of the outer head ; 

 the ulnar head of the index 

 flexor becomes the first palmar 

 interosseous, the radial heads 

 of the short flexors of the ring 

 and little fingers form the 

 second and third palmar inter- 

 osseous muscles, and the ulnar 

 head of the little finger muscle 

 is represented only by the small 

 part of the opponens minimi 

 digiti deeper than the nerve ; 

 the radial head of the flexor 

 indicis, both heads of the flexor 

 medii, and the ulnar head of 

 , the flexor annularis are absent. 



The deep layer is represented in the abductors of the thumb and little finger, with the 

 four dorsal interosseous muscles, two of which become inserted into the middle finger. 

 In the foot, while the general disposition of the muscles is similar, there are the 

 following differences in detail : The adductor muscle of the little toe is also sup- 

 pressed, and the flexor brevis and opponens are formed from the fibular head of the short 

 flexor ; both heads of the short flexor of the second toe are absent, and the first plantar 

 interosseous is the inner head of the short flexor of the third toe, while the second dorsal 

 interosseous muscle becomes inserted into the fibular side of the second toe. (D. J. Cunningham, 

 Journ. Anat., xii and xiii, and " Challenger " Reports, Zoology, v ; H. St. John Brooks, 

 Journ. Anat., xx.) 



The chief points of importance in seeking- to determine muscular homologies are position, 

 attachments and nervous supply. With reference to these it may be remarked : 1. The 

 position of a muscle may be changed, and it may thus acquire an action differing from that of 

 the mass from which it is derived. Thus, the supinator longus undoubtedly belongs to the 

 dorsal or extensor mass of the upper limb, but being placed in front of the elbow it becomes 

 a flexor of that joint ; similarly, the peronei, longus and brevis, belonging to the proper 

 dorsal group of the leg, but passing behind the external malleolus, produce movement of the 



EXT. PLANTAR NERVE 



