286 SHOULDER-GIRDLE IN MONOTREMES, 



nerve of the thorax. It is, therefore, quite possible that although 

 the muscle is chiefly supplied by the external anterior thoracic, it 

 may, however, get some sui)ply from the lateral cutaneous nerve. 

 And indeed this vfould seein to be the case, for we find, in looking 

 through some MS. notes of Professor Wilson, the following 

 passage — " I have found the P. quartus wholly supplied in Or7d- 

 thorhyyichus by the lateral cutaneous nerve of the thorax." 

 Furthermore, Smith has found the P. quartus in Ornithorhyyichus 

 supplied by a nerve coming from two roots, one from the external 

 anterior thoracic, the other from the lateral cutaneous nerve of the 

 thorax. 



Westling also found that there was a connection between the 

 nerve to the P. major and the lateral cutaneous nerve. She 

 says — "Vom C. vii. und viii. und von dem vom D. i. und ii. 

 enstandenen Stamme geht ein sehr starker Nerv [vide Fig. 17) aus, 

 der den untern Theil des M. pectoralis major versieht und sich 

 ausserdem in Haut- oder Hautmuskelnerven vertheilt : wahr 

 scheinlich ist es ein N. thoracicus anterior." 



Westling also remarks, in giving an account of the brachial 

 plexus in Echidna, " der Nerv zu letzterm Muskel [M. pectoralis] 

 verbindet sich mit dem einen der Hautmuskelnerven (Fig. 15, 54) 

 die aus den hintern Wurzeln des Plexus brachialis entstehen." 



Taking the above facts into consideration, it appears to us that 

 we are quite justified in regarding the muscle under discussion as 

 a pectoralis quartus ; and if a further explanation of the nerve 

 supply were wanted, we think that the following points from Pro- 

 fessor Wilson's notes would aid us in comprehending the true 

 nature of the P. quartus. Professor Wilson says — " I think it 

 most probable that various severally distinct muscular sectors in 

 the posterior region of the Mammalian thoracic, or from the abdo- 

 minal, wall, have received the common name of pectoralis quartus; 

 and that a whole series of transition forms exists between a con- 

 dition, {a) in which a true pectoralis quartus appears as simply a 

 posterior sector of the general pectoral mass arising from the 

 mesio-ventral line, and wholly or largely distinct from a humeral 



