Anatomy, histology and innervation of the heart of spiny lizard, U. hardwickii have been described. The heart is typically reptilian and comprises four chambers: sinus venosus, two atria and a ventricle. The sinus venosus is a small, thin-walled chamber present on the dorsal side of right atrium. The sinuatrial opening is protected by a pair of valves. The auriculoventricular opening is also protected by a pair of muscular valves. The presence of a cardiac conducting system is reaffirmed. The conducting system comprises sinuatrial and atrioventricular nodes. The sinuatrial node is present at the sinuatrial junction very close to the interauricular septum. The atrioventricular node is quite conspicuous and located at the caudal end of the interauricular septum. The atrioventricular bundle is absent. There is present an atrioventricular cushion which serves the purpose of atrioventricular bundle. The heart is richly innervated, as is evident by the presence of a large number of nerve fibres and ganglion cells at the different levels of the heart. Both the nodes and the atrioventricular cushion are supplied by the fine nerve fibres, but ganglion cells could not be found in any one of these components. The myogenic theory of conduction systems is supported.

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