CT and Cross-sectional Anatomy of the Normal Head of the Loggerhead Sea Turtle (Caretta caretta)

The third eyelid protects the eye, contribute to the aqueous portion of the pre-ocular film and its distribution over the corneal surface, as well as removal of foreign material from the anterior surface of the eye. It is rather vulnerable during fights and clinicians often have to perform surgery for third eyelid neoplasia. In dogs, this lid has a triangular shape, being widest at its free margin. It is vascularized by branches of the malar artery. Being of such importance as an adnexa of the eye, the authors concentrated their attention on its vascularization, which is not well defined.
Methods:  Scanning Electronic Microscopy of vascular corrosion casts and intact tissue.
Observations:  It was quite interesting to notice the presence of two patterns of vascularization inside the third eyelid. One can say that there are two segmental levels of vessels. In one of these there is a predominance of arteries and veins, while in the other a meshwork of third order arteries, capillaries and post-capillary venules are present. In fact, the branch of the malar artery that reaches the base of the third eyelid divides into smaller branches that cross almost the complete length of the lid giving only a few collateral vessels, in a monopodical way, and only near the free margin of the lid is possible to observe dicothomical ramifications that became rather anastomotic (being anastomoses of type I and II are present) and contribute to the microvascular meshwork. This microvascular net is drained by post-capillary venules and veins that empty as a brush into bigger veins that occupy the same segmental level as the third eyelid artery.
Remarks:  This uncommon angioarchitecture is probably an adaptation to the fact that in the medial canthus of the eye this lid has a smaller space and the way of obtaining an efficient vascularization is by going straight to the top of the lid.