1868.] Mr. J. Wood on Variations in Human Myology. 505 



the insertion of this outer tendinous shp from the longior was found in 

 both arms, and was so significant tliat it has been chosen as the subject of 

 fig. 6. Leaving the outer side of the longior tendon and but shghtly in- 



Fig.6 

 (Subject No. 4). 



ferior to it in size, just above the radial styloid process, 



it crossed the depression of the " tahatiere'^ under the 



extensor tendons of the thumb, and reached the first 



interosseous space. There it subdivided into two slips, 



the outer one of which was inserted into the inner part 



of the base of the ^oZZe^-metacarpal (a) ; and the inner, 



spreading out into a sort of aponeurosis, was first at- 

 tached to the base of the 2??cZe.r-metacarpal (h), and then 



passed into the united origins of the abductor indicts 

 (interosseus prior indicts of iVlbinus) (c), and of an 



interosseus primus volaris of Henle (c?). The first dor- 

 sal interosseus was entirely divided into two muscles, of 

 which the posterior (e) arose from the contiguous meta- 

 carpals quite distinctly from the deeper muscle (c), 

 which also arose by a bifurcated origiuj one from the 

 index -metacarpal, and the other from the slip of the 

 accessorius tendon under description, in common with 

 the interosseus volaris. In the figure, the dorsal por- 

 tion is cut off close to its two origins to show the deeper 

 part. In the same arm was observed an extensor-zw^er- 

 medius tendon, also leaving that of the longior (/), 

 rather higher than the accessorius, and joining that of 

 the hrevior (g) at its insertion into the second and third 

 metacarpals. The thumb-extensors are in the figure 

 cut off close to their origins and insertions. 



In this specimen we have clearly some light thrown 

 upon the way of the formation of the anomalous exten- 

 sor accessorius. The abnormal muscle is produced 

 simply by lateral differentiation and displacement of the 

 outer part of the muscle and tendon of the longior, a 

 process which stops, in the specimen just described, at 

 -the tendon only. 



In the left arm of the male (No. 6) a similar shp from the longior was 

 inserted into the base of the pollex-metacarpal, and was continuous with 

 the deep origin of the jlexor brevis pollicis, there being no interosseus 

 volaris present. In the right arm of No. 10 the same abnormal tendon 

 was even larger than the normal one of the longior, and there were decided 

 marks of a division of the muscular belly into a distinct muscle. The 

 accessory tendon divided about an inch above its termination into an inner 

 slip inserted into the outer tubercular projection of the base of the index- 

 metacarpal, and an outer one which subdivided ; one of the subdivisions, 

 being inserted into the base of the pollex-metacarpal, gave part origin 



