PLATE XVI. 

 THE DEEP DISSECTION OF BACK OF FOREARM. 



In this dissection the extensores communis digitorum, minimi <ligiti, and carpi 

 ulnaris have been cut away to expose the deep layer of muscles in the forcann, and the 

 posterior interossi IH< ud nerve. 



Note on the Extensor nn-iJins. A so] orate tendon to the middle finger is not 

 uncommon ; it usually arises by a muscular belly attached to the posterior surface of 

 tlir ulna lie-low the extensor indicis ; its tendon runs down through the same com- 

 partment of the annular ligament as the extensor communis digitorum, but deep to it, 

 and joins the common extensor tendon of the middle finger in the same way as the 

 extensor indicis joins with that of the index finger. 



Thus the extensor tendons may be divided into two sets a common set, which 

 is superficial (the extensor communis digitorum); and a special set, of which the 

 extensor minimi digiti is superficial and constant, the extensor indicis deep and 

 lit, the extensor medius deep and fairly frequent; the only one missing being 

 the extensor annularis, which is very rare ; but, as Mr. Button has pointed 

 out, if the fascia of the forearm be carefully examined, a thin glistening baud of 

 tendinous fibres will be seen lying in it, in the intermuscular septum between the 

 extensor carpi ulnuris and the extensor minimi digiti, arising in common with the 

 In tier, aud passing down to become lost in the fibrous tissue at the back of the wrist. 

 This, in Mr. Button's opinion, is probably the degenerate remains of the extensor 

 Bimularis, which in process of shifting its origin from the condyle of the humerus to the 

 ulna has become lost. This opinion is strengthened by the fact that the extensor minimi 

 digiti does not arise by muscular fibres from the condyle, but takes its origin from the 

 intermuscular septum between the extensor carpi ulnaris and the extensor communis 

 digitorum, as though this muscle also was in process of descending from its condylar to 

 some lower attachment. 



.The so-called ganglion on the posterior interosseous nerve is only a thickening of 

 its perineurium. 



