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Show 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 Heloderma 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 they 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 corresponding 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-maadi-bularis that we discover the subelliptical periphery 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 the skin, where anteriorly it is attached, being in contact above with the poison-gland of the corresponding side. N o w 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. Pteryyoideus internus.-More modest in its proportions than the last described, this muscle finds its origin upon the outer margin of the parietal bone, aud the adjacent surface of the prootic, at the antero-external region of the orbit. From this point of origin its fibres 12* |