THE HEAD AND NECK. 149 



Describe the sixth cranial nerve. 



It is called the abducent nerve. It enters the orbit by the sphenoidal fissure, 

 between the two heads of the external rectus, to which muscle it is distributed. 



Describe the optic nerve. 



This is the nerve of the special sense of sight. It is the second cranial nerve. 

 It enters the orbit by the optic foramen, internal to and above the ophthalmic 

 artery. It pierces the ball of the eye and forms the retina. It is surrounded by 

 the ciliary vessels and nerves. To its outer side is the ciliary ganglion. At the 

 optic foramen it is surrounded by the four recti muscles. The arteria centralis 

 retinae pierces its under surface and goes to the interior of the eyeball. The 

 process of dura that surrounds the nerve divides in such a manner as to form a 

 sheath for the nerve, and also the orbital periosteum, inside the orbit. The 

 optic sheath is continuous with the sclerotica. 



Wliat is the orbital fascia? 



It is a variety of connective tissue, being to the contents of the orbit what 

 the deep fascia is to the thigh. It forms a capsule, in conjunction with the peri- 

 orbita, for the lachrymal gland, and sheaths for the muscles, vessels, and nerves. 

 It is connected to the ocular conjunctiva close to the cornea. The muscular 

 sheaths are firmly adherent to the muscles. The insertions of the ocular muscles 

 into the sclerotica are connected together around the whole circumference of 

 the globe. From this attachment this fascial connecting medium is reflected 

 backward in a double layer, forming the fibrous basis of a serous membrane 

 the visceral layer covering the back part of the globe, the parietal lining the 

 postocular fat in the orbit. A layer of endothelial cells completes the serous 

 membrane. Here we have a visceral layer, a cavity, and a parietal layer, the 

 three cardinal points in any serous apparatus. This reflected part of the orbital 

 fascia permits free movement of the eyeball, as the head of a bone revolves in its 

 socket, and is known in anatomy as the capsule of Tenon. 



By what is the orbital fascia on the posterior part of the eyeball pierced ? 



It is pierced by the vessels and nerves that supply the globe. 



What muscle did you find immediately under the levator palpebrce ? 



The superior rectus, and under this we found the optic nerve, surrounded by 

 fat, vessels, and nerves. 



Name the muscles of the eyeball and give their function. 



The external rectus abducts the cornea. 



The internal rectus adducts the cornea. 



The superior rectus elevates, adducts, and rotates cornea inward. 



The inferior rectus depresses, adducts, and rotates cornea outward. 



The superior oblique depresses, abducts, and rotates cornea inward. 



The inferior oblique elevates, abducts, and rotates cornea outward. 



The levator palpebrae elevates the upper eyelid. 



Name all the nerves that supply the orbital contents. 



The optic, the nerve of special sense of sight. 



The motor oculi, a motor nerve to all muscles except two. 



The pathetic, a motor nerve to the superior oblique muscle. 



The abducens, a motor nerve to the external rectus muscle. 



The frontal, a branch of the ophthalmic of the fifth nerve. 



The lachrymal, a branch of the ophthalmic of the fifth nerve. 



The supratrochlear branch of the frontal. 



The palpebral branches of the frontal. 



The infratrochlear branch of the nasal. 



The long ciliary branches of the nasal. 



The short ciliary branches of the lenticular ganglion. 



The cavernous sympathetic branches to lenticular ganglion. 



