HEAD AND NECK OF THE HORSE 107 



Dissection. — Remove the temporal muscle. Saw across the zygo- 

 matic process of the frontal bone, immediately lateral to the supra-orbital 

 foramen, and the zygomatic bone on a level with the maxillary tuber. 



Observe the considerable collection of fat— varying in amount with 

 age — that lies between the border of the temporal muscle and the 

 periorbita. 



The orbit (Orb! ta).— Although the orbit is usually described as a 

 fuunel-shaped cavity, with its apex at the optic foramen and its base at 

 the rim of bone formed by the frontal, lachrymal and zygomatic, in the 

 macerated skull its boundaries are far from definite ; for, except for a 

 medial Avall and a ring of bone in front, the cavity is undefined and 

 communicates freely with the temporal and pterygo-palatine fossae. 



The dissector will find it profitable to make an examination of the 

 orbit of a macerated skull before proceeding to the dissection of the 

 contents of the cavity. The base or inlet of the orbit (aditus orbitse) is 

 circumscribed by the zygomatic process of the frontal, the zygomatic 

 process of the temporal, the temporal process of the zygomatic, and the 

 lachrymal bone. The apex is at the optic foramen. The axis of the 

 orbit (axis orbital), taken as passing from the optic foramen to the 

 central point of the inlet, runs in a forward, lateral and slightly upward 

 direction. The superior wall (paries superior) is formed mainly by the 

 frontal bone, and to a certain extent by the lachrymal also. The medial 

 wall (paries medialis) is the only one that is completely formed by bone, 

 and the frontal, lachrymal, and orbital wing of the sphenoid enter into 

 its formation. Close to the rim of the orbit, and in the lachrymal 

 bone, there is a fossa (leading into a foramen) for the reception of the 

 lachrymal sac; and immediately behind this is a rounded depression 

 from which the inferior oblique muscle of the eyeball arises. The 

 inferior wall (paries inferior) is very incomplete, and is formed by the 

 zygomatic process of the temporal, the temporal process of the zygo- 

 matic, and a small part of the maxilla. The lateral wall (paries lateralis) 

 consists of the zygomatic process of the frontal bone. The superior and 

 lateral walls are smoothly continuous, and a shallow depression in which 

 the lachrymal gland is lodged (fossa glandulas lacrimalis) occurs on the 

 under surface of the zygomatic process of the frontal. 



Though the bony boundaries of the orbit are so incomplete, the 

 definition of the cavity in the recent state is rendered precise by the 

 presence of the periorbita. 



The periorbita is a stout, inelastic, fibrous sheath of conical form, 

 with an apex adherent to the bone around the optic foramen and the 

 orbital fissure, and a base that merges partly into the periosteum of the 

 bones that bound the orbital inlet and partly into the fibrous tissue of 

 the eyelids. In thickness the sheath is far from uniform. Thickest 



