BY W. J. S. McKAY, 285 



Tlie origins and insertions of the muscles in Echidna and Orni- 

 thorhynchus ax-e practically identical. The relations vary slightly, 

 inasmuch as the postero-internal Vjorder of the deltoid in Echidna 

 is placed, in part, distinctly superficial (ventral) to the P. major 

 (Fig. 3, Dlt. C), while in Ornithorhynchus the two muscles are in 

 the same plane, with a distinct cellular interval between their 

 borders. Lastly, and most important of all, the nerve supply to 

 the muscles in both animals is identical, and is derived from the 

 representative of the circumflex nerve (N. axillaris), which nerve 

 also supplies the scapular portion of the deltoid, about the homo- 

 logy of which there is no dispute. 



As regards the muscle described by us above as pectoralis 

 quartus, it appears that this name has not been used in connection 

 with this muscle by any previous writer with the exception of 

 Smith in his description of the pectoral muscles in Ornithorhynchus, 

 which we have quoted above. Galton mentions en passant in a 

 description of the pectoral muscles of Dasypus (8), that in Or^ii- 

 thorhynchus and Echidna " the posterior abdominal portion was 

 naturally differentiated from the pectoral factor of the muscle," 

 but does not call it P. quartus. Windle (23) does, however, appear 

 to regard this muscle as P. quartus. In attempting to decide the 

 point whether this muscle is P. quartus, or merely a posterior portion 

 of the P. major, we can say that the origin, insertion, and relations 

 are very similar to those described for P. quartus in other animals. 

 But in examining the nerve supply we have found that both 

 in Echidna and Ornithorhynchus the muscle is supplied by a branch 

 which comes from the nerve to the P. major, i.e., the external 

 anterior thoracic. But according to Windle (23) the P. quartus 

 should be supplied by the posterior pectoral nerve, i.e., the lateral 

 cutaneous nerve of the thorax (N. of Wrisberg, Patterson) ; or less 

 commonly as pointed out by Bermingham (1) by the middle pec- 

 toral, i.e., the internal anterior thoracic. It will be seen by 

 referring to our figure of the brachial plexus of the Echidna (Fig. 

 13) that the nerve to the P. quartus, after leaving the nerve to 

 the P. major, gets a communication from the second intercostal 

 nerve, which in turn communicates with the lateral cutaneous 

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