BY W. J. McKAY, 935 



two heads coalesce into a rounded belly of muscle which runs 

 forward and downwards, the lowermost fibres being inserted on to 

 a special process in the middle of the inferior surface of the 

 transverse bone, and also into the posterior half of that bone. 

 The upper fibres, (or those that mainly spring from the internal 

 surface of the mandible), are inserted into the inferior surface of 

 the posterior half of the pterygoid bone. The muscle at its origin 

 has the digastric above it externally, while the spheno-pterygoid 

 lies on the internal face. Inferiorly the muscle is completely 

 covered by the mylohyoid. 



The muscle is usually described as two, i.e., the external and 

 internal pterygoids. We have carefully dissected several forms 

 to ascertain if there are any grounds for this separation, and we 

 find that the separation into two muscles is quite unnecessary. 

 The same holds good for Hydrosaurus. We may also add that 

 the same nerve supplies both parts of the muscle. 



In Dahoia and in all vipers the muscle is prolonged forward so 

 as to be inserted into the maxilla, and at the same time sending a 

 tendon to act on the mucous membrane that covers the fangs. 

 This arrangement of the pterygoid muscle forms a valuable means 

 of distinguishing the vipers from the venomous colubrine snakes. 

 The insertion of the muscle in Acantliophis into a special process 

 on the transverse bones is interesting, as a similar process for its 

 insertion is seen in Hydrosaurus. 



M. DiGASTRicus (Posterior belly). 



M. occipito-quadrato-mandibularis, Hoffbaann ; Niederzieher 

 des UnterkieferSj D' Alton ; M, temporalis^ von Teutleben ; M 

 tympano-manihularis, Owen; M. digastricus, R. Jones, Duvernoy. 



The digastric arises by two distinct portions. The smaller one 

 springs from the ridge between the supraoccipital and the epiotic 

 bones, and from the posterior third of the superior surface of the 

 squamosal ; the larger portion arises from the posterior surface of 

 the quadrate, and from the capsule of the joint between the 

 quadrate and the squamosal. The bellies coalesce above and run 

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