1890.] HELODERMA SUSPECTUM. 159 



a manner already alluded to. These two muscles seem to constitute 

 the platysma myoides of Sanders, and from this it will be seen that 

 Hehdernia suspectum is one of those lizards wherein the mylo- 

 hyoidean muscles are conspicuously developed. By removing them 

 we at once expose the deeper set of the true muscles of the hyoid 

 arches. 



3. Temporalis. — Notably dense and bulky, this is one of the, if 

 not the, most powerful muscles of the system to be found in this 

 lizard's economy. It takes origin from the parietal, from the 

 nether surface of the squamosal and postfrontal, and from the 

 anterior aspect of the quadrate ; from this extensive surface its fibres 

 rapidly converge as they pass downwards and forwards, and becoming 

 strongly tendinous tliey make insertion upon the postero-external 

 border of the coronary process of the mandible ; this constitutes 

 its chief insertion, but beyond this its tendon also fastens itself 

 to the outer surface of the coronary, extending to the corre- 

 sponding aspect of the adjacent side of the mandible as far forward 

 as the dentary element, and posteriorly to a slight extent towards the 

 hinder end of the bone. It will be seen from this that the muscle 

 quite fills the temporal fossa, its insertional margin being limited 

 sharply above by the cranial bones that go to form the outer 

 edge of the orbit ; and it is between the posterior border of this 

 muscle and the anterior margin of the digastric and neuro-mandi- 

 bularis that we discover the subelliptical perij)hery of the ear, with 

 its tympanic membrane tightly stretched over it. 



In making my dissections of these parts, I find an interesting 

 tendon which arises from near the posterior end of the mandible, 

 and passing directly forwards, on a line with the upper edge of the 

 lower jaw, commences to expand about opposite the coronoid process of 

 that bone, and spreading out like a fan, thence on becomes intimately 

 attached to the antero-lateral skin of the throat. It is superficial to 

 all these structures, except of course tlie skin, where anteriorly it is 

 attached, being in contact above with the poison-gland of the corre- 

 sponding side. Now there are a few muscular fibres to be seen in 

 the anterior portion of this tendon, so that upon contraction it will 

 tend to press with some force the poison-gland against the mandible, 

 and thus be auxiliary to forcing its secretion through the gland's 

 ducts at the time this reptile makes its bite. 



4. Pterygoideus externus. — Chiefly carneous, this is another large 

 muscle of this region, which arises from the inferior aspect of the 

 corresponding pterygoid to pass backwards and outwards in the 

 form of a subelliptical bundle of fleshy fibres that take it upon 

 themselves to completely envelop the articular extremity of the 

 mandible, being inserted into the three elements that go to form 

 that end of the bone. This muscle is somewhat tendinous at its 

 origin, but almost entirely carneous at its insertion. 



.5. Pterygoideus internus. — More modest in its proportions than 

 the last described, this muscle finds its origin upon the outer margin 

 of the parietal bone, and the adjacent siuface of the prootic, at the 

 antero-external region of the orbit. From this point of oriain its fibres 



12* 



[13] 



