11. Traditionally, the term has been defined on the basis of the shape of the infraorbital canal. 12. Unlike hystricomorphous and myomorphous rodents, the medial masseter muscle does not pass through the infraorbital canal. 13. The "'infraorbital canal "'is a orbit that opens on to the maxilla. 14. Frequently involved structures include the lacrimal glands, extraocular muscles, infraorbital nerve, supraorbital nerve and eyelids. 15. Alternatively, some texts group rodents into three suborders on the basis of the shape of the infraorbital canal. 16. This is related to the accumulation of blood and other fluid in the infraorbital groove resulting from nasal congestion. 17. IONE is defined as the infraorbital nerve diameter being greater than the optic nerve diameter in the coronal plane. 18. This condition is so pronounced and the snout so narrow in heteromyids that the infraorbital canals from either side connect. 19. The infraorbital canal is enlarged, presumably allowing for the passage of the medial masseter muscle as with other hystricomorphs. 20. Above the fossa, the infraorbital canal ends in the infraorbital foramen an opening that transmits the infraorbital vessels and nerve.
