The swordfish ﹛Xiphias gladius), as well as billfishes (Istiophoridae), shows a conspicuous modification of one of its eye muscles: the dorsal rectus muscle. While the other eye muscles show an apparently normal, oculomotoric structure, the musculus rectus dorsalis is modified for heat production. While the remaining oculomotoric portion of this muscle shows normal muscular cells, the bulk of the organ is composed of cells structurally similar to brown adipose tissue (BAT) and blood vessels. The BAT cells are capable of direct (i.e., non-shivering) metabolic heat production. This heat is then transported and disseminated by the blood vessels in this organ.

Microvascular corrosion casting is an excellent technique for studying the morphology of complicate vascular networks. However, studies using this technique on eye muscle vasculature are not frequently found in the literature. Juvenile swordfish, 1.4-1.8kg bodyweight, were collected in the Ionic sea. Immediately after catching, terminal anaesthesia was carried out with tricaine-methanesulfonate.