The carotid and orbital retia of the pronghorn, deer and elk

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Abstract
Selective cooling of the brain during heat stress has been shown by others to be a method of temperature regulation for mammals having carotid retia. This study describes the macroscopic anatomy of the cranial circulation of elk, deer and pronghorn as it might pertain to the functioning of carotid retia and orbital retia as heat exchangers. Emphasis has been placed on describing the source of venous blood bathing these retia, for blood flow from these sources to the ophthalmic plexus and cavernous sinus will establish a temperature difference between arterial and venous blood, and influence the magnitude of this gradient. The pronghorn possesses a carotid rete with greater density and smaller calibre vessels overall and a more highly vascular orbital rete compared to the elk and the deer. These anatomical differences may indicate differences in efficiency of heat exchange in the retia. It is suggested that the orbital rete is anatomically in a position to moderate extremes of temperature by cooling arterial blood flowing to neural tissue of the eye and olfactory bulbs.