240 Mammalia. 



how mucli bone he will have to saw in each case, and will 

 take care not to cut too deep ; to avoid which, it is always 

 best to finish the fracture by the introduction of a chisel into 

 the groove made, and forcing the crack by a sort of lever 

 action on the bone surfaces. The two pieces of hone included 

 between these three incisions should he removed after that the 

 contents of the orbital cavity have been detached from all 

 connection with their inner surface. Special care will be 

 needed as the dissector approaches near to the optic foramen, 

 to avoid injuring the Fourth nerve. 



The Orbit contains the following structures within the 

 orbital membrane, viz. : the Eyeball, the Lachrymal, and 

 Harderian glands ; six muscles connected with the eye 

 four straight and two oblique : and one muscle connected 

 with the upper eyelid : also numerous nerves, viz., the Second, 

 Third, Fourth, Ophthalmic division of the fifth, and the Sixth, 

 with part of the Sympathetic : and, lastly, the Suspensory 

 (or Choanoid) muscle. 



Some of the mass of fat which fills up the lower part of 

 the orbital cavity may be removed, care being taken not to 

 cut down on the Superior Maxillary branch of the Fifth nerve, 

 which runs horizontally forward from the inferior limit of 

 the sphenoidal fissure. 



It will then be found that the lower part of the orbit, 

 which is completed in Man by an osseous framework, is here 

 only completed by membrane. This membrane is the orbital 

 membrane or periosteum, which thus becomes a structure of 

 much importance, for it stretches across the floor of the orbit 

 from its outer to its inner wall, extends backwards to the 

 optic foramen, and completes the boundary of the cavity at 

 the spot where the bony wall is wanting. Intimately connected 

 with the membrane, and forming an essential part of it, is a 

 thin layer of a pale-reddish substance, which extends across 

 the greater part of the floor of the orbit, passing backwards 



