The =orbit= is a cavity which incloses the eyeball, with the muscles, vessels, and nerves associated with it. It is not separated in the skeleton from the temporal fossa. The =long axis= of the cavity, taken from the optic foramen to the middle of the =inlet=, is directed forward, outward, and slightly upward. The =inner wall= (Paries medialis) is complete and extensive. It is concave and smooth, and is formed by the frontal and lacrimal and the orbital wing of the sphenoid. In its extreme anterior part is the fossa for the lacrimal sac. Behind this is a small depression in which the inferior oblique muscle of the eye arises; here the plate which separates the orbit from the maxillary sinus is very thin. The =upper wall= (Paries superior) is formed by the frontal and to a small extent by the lacrimal bone. It presents the supraorbital foramen, which perforates the root of the supraorbital process. The =lower wall= (Paries inferior) is very incomplete, and is formed by the malar, the zygomatic process of the temporal, and to a small extent by the maxilla. The =external boundary= (Paries lateralis) is the supraorbital process. At the extreme posterior part is the =orbital group of foramina=. Four are situated in front of the pterygoid crest. Of these, the uppermost is the =ethmoidal= or =internal orbital foramen=, which transmits the ethmoidal vessels and nerve. The =optic foramen= is situated a little lower and further back; it transmits the optic nerve. Immediately below the optic is the =foramen lacerum orbitale=, which transmits the ophthalmic, third, sixth, and sometimes the fourth nerve; commonly there is a very small =trochlear= or =pathetic foramen= in the crest for the last-named nerve. The =foramen rotundum= is below the foramen lacerum, from which it is separated by a thin plate; it transmits the superior maxillary nerve. The =alar canal= opens in common with the foramen rotundum, and the anterior opening of the pterygoid or Vidian canal is also found here. The =temporal foramen= (For. alare parvum) is just behind the pterygoid crest and on a level with the foramen lacerum. It is the upper opening of a canal which leads from the alar canal, and through it the anterior deep temporal artery emerges. The =inlet= of the orbital cavity (Aditus orbitæ) is circumscribed by a complete bony ring, which is nearly circular. Its antero-inferior part (Margo infraorbitalis) is smooth and rounded; the remainder (Margo supraorbitalis) is rough and irregularly notched. During life the cavity is completed by the periorbita or ocular sheath, a conical fibrous membrane, the apex of which is attached around the optic foramen.