82 APPLIED ANATOMY. 



within, causing thrombosis and death. The inferior ophthalmic usually empties into 

 the superior; its anastomoses at the anterior portion of the orbit with the veins of 

 the face are much smaller and, therefore, not nearly so dangerous. 



Nerves of the Orbit. The optic nerve is the nerve of sight. Interference 

 with it produces blindness. The ocidomotor or third nerve supplies all the muscles 

 of the orbit except the external rectus and superior oblique. If paralyzed, the eye 

 cannot be moved upward, inward, or to any extent downward. There will be ptosis 

 of the upper lid from paralysis of the levator palpebrae, and dilatation of the pupil and 

 paralysis of the accommodation of the eye. If the sixth or abducens is paralyzed, the 

 eye cannot be turned outward. If the fourth or pathetic is paralyzed, the superior 

 oblique fails to act, and the double vision produced is worse when the patient looks 

 down, because it is normally a depressor muscle. The lachrymal, frontal, and nasal 

 branches of the fifth are nerves of sensation, hence, in supra-orbital neuralgia and that 

 affecting the nasal branch, pain is felt in the orbit at the inner angle of the eye and 

 down the side of the nose. 



Retina. On the interior of the eye, the expansion of the optic nerve forms the 

 retina. The retina is divided into two lateral halves, each supplied by a corre- 

 sponding half of the optic nerve. When the nerve reaches the optic chiasm it splits 

 into two parts, one (internal fibres) going to the opposite side of the brain, and 

 the other (external fibres) to the ganglia on the same side of the brain. Posterior 

 to the chiasm, the nerve fibres form the optic tracts. The optic tracts, after leaving 

 the chiasm, wind around the crura cerebri to the external geniculate bodies, thence 

 they pass to the thalami and anterior corpora quadrigemina, and are continued back- 

 ward into the cuneus lobule of the occipital lobe of the brain. 



It will thus be seen that a lesion affecting any portion of the optic pathway pos- 

 terior to the chiasm will produce blindness of one-half of the retina of both eyes on 

 the side of the injury; a right-sided lesion will produce blindness of the right half of 

 both retinae, and a lesion on the left side, blindness of the left half of both eyes. This 

 is called hemianopia. It is right lateral hemianopia if the right half of the visual fields 

 is affected, and left lateral if the left sides are affected. Affections of the optic nerve 

 produce total blindness of that eye if the whole nerve is involved. If only a part is 

 involved, then a unilateral hemianopia may ensue. A bitemporal hemianopia may be 

 caused by a tumor involving the anterior or middle portion of the chiasm. A binasal 

 hemianopia requires a symmetrical lesion on the outer side of both optic nerves or 

 tracts. A brain tumor located in the cuneus lobule would cause a lateral hemianopia of 

 the same side, right or left, of both visual fields, hence sometimes called homonymous. 



The Eyelids and Conjunctiva. The eyelids are composed of five layers, 

 viz : (i) skin, (2) s^lbcutaneous tissue, (3) orbicularis palpebrarum muscle > (4) 

 tarsal cartilage with the contained Meibomian glands, (5) the conjunctiva. The 

 juncture of the two lids at each end is called the inner and outer canthus. 



The skin of the lids is thin and the subcutaneous tissue loose and devoid of fat. 

 For these reasons blood finds its way readily into the lids and shows plainly beneath the 

 skin, constituting the familiar "black eye." The skin lends itself readily to plastic 

 operations, as it is easily raised and the gap left can be readily closed. The blood 

 supply of the lids is abundant, so that the flaps are well nourished and sloughing is 

 not apt to occur. The folds in the skin run parallel to the edge of the lids, therefore 

 the incisions should be made as much as possible in the same direction. The 

 orbicularis palpebrarum muscle passes circularly over the lids and lies on the tarsal 

 cartilage toward the edge of the lids and on the orbitotarsal ligament above. The so- 

 called tarsal cartilage or plate is composed of dense connective tissue and contains no 

 cartilage cells. It is attached externally by the external {lateral} palpebral ligament 

 and internally by the internal (medial) palpebral ligament or tendo-oculi. This latter 

 passes in front of the lachrymal sac. The tarsal plate is continued to the rim of the 

 orbit by the orbitotarsal ligament or septum orbitale. The expansion of the levator 

 palpebree muscle ends in the upper edge of the tarsal cartilage and sends some fibres 

 to the tissues immediately in front. The orbitotarsal ligament and tarsal cartilage 

 prevent the fat of the orbit from protruding and also act as a barrier to the exit of pus. 



The tarsal cartilage contains the Meibomian glands. These can be seen in life, 

 by everting the lid, as yellow streaks passing backward from the edge of the lids. 



