134 DISSECTION OF THE DOG 



of the muscle passes down that groove of the radius in which the common 

 extensor tendons he. In the metacarpal region it divides into two parts : 

 one (inconstant) joins the first digit (m. extensor pollicis longus) ; the other 

 fuses with that shp of the common extensor which goes to the second digit 

 (m. extensor indicis proprius). 



M. ABDUCTOR POLLICIS LONGUS.— The abductor of the first digit is distinctly 

 semi-penniform. Its origin, partly under cover of the foregoing muscle, is from 

 the middle two-fourths or more of the adjacent borders of the radius and ulna 

 and the intervening interosseous membrane. The tendon runs along the 

 medial border of the muscle, crosses obliquely over the tendon of the radial 

 extensor, and ends on the base of the first metacarpal bone. A sesamoid bone 

 is associated Avith the tendon at its insertion. 



M. SUPINATOR. — The supinator muscle is small, flat, and irregularly 

 triangular, and lies on the proximal fourth of the radius. Arising from the 

 lateral epicondyle of the humerus and the lateral collateral ligament of the 

 elbow-joint in association with the common origin of the extensors of the 

 third, fourth, and fifth digits, the muscle is inserted into the anterior surface 

 and medial border of the radius partly under the insertion of the pronator 

 teres muscle. 



N. RADiALis PROFUNDUS. — In the dissection of the arm it was seen that the 

 radial nerve divided into deep and superficial branches. The deep radial nerve 

 crosses the flexor aspect of the elbow-joint under cover of the radial extensor 

 and supinator muscles. Inclining laterally, it then passes between the radius 

 and the extensors of the third, fourth, and fifth digits to end in the ulnar 

 extensor of the carpus. 



Dissection. — Before removing the skin from the palmar aspect of the manus, 

 observe the presence of callosities similar in position and form to the five 

 eminences encountered in the pelvic limb. In addition there is a sixth 

 callosity present in a line with, but immediately distal to, the prominence 

 caused by the pisiform bone. When the callosities have been examined 

 the skin should be removed and the characters and connections of the 

 fascia noted. 



Fascia. — The fascia at the back of the forearm may be divided into two 

 layers. The more superficial of the two is the looser, and is continuous with the 

 fascia of the arm. The deeper and denser layer is more limited to the forearm 

 itself. It forms a tough investment for the muscles of the region, and sends 

 septa bet-ween the individual members of the group. The fascia of the forearm 

 is especially strong and tendinous in character close to the carpus. Here a 

 band of it passes under the annular ligament (from the carpus to tendon of 

 superficial flexor of the digits) and is partly attached to the medial border of 

 the carpus and partly continued to the dorsal aspect of the manus. Over the 



