1440 HUMAN ANATOMY. 



are controlled chiefly by the superior and inferior recti respectively, but each has a 

 slight adducting and a slight rotating movement i.e. , the superior rectus will move 

 the upper extremity of the vertical meridian slightly inward (intorsion), and the 

 inferior rectus will move the same part slightly outward (extorsion). The superior 

 oblique is attached to the globe behind the equator, and lower than its pulley, so that 

 in addition to its chief or internal rotating action upon the upper limit of the ball it 

 has also an elevating effect upon the posterior portion, the cornea moving down- 

 ward. Since its pull is inward, the cornea also moves outward. The chief move- 

 ment of the inferior oblique is rotary in the opposite direction (extorsion of the upper 

 part). It is likewise inserted into the posterior half of the globe, which is depressed 

 by it, and the cornea is raised and moved outward. In elevation of the cornea by 

 the superior rectus the internal rotation of this muscle is counteracted by the inferior 

 oblique, and in a similar manner when the cornea is moved downward by the inferior 

 rectus, its external rotation is opposed by the superior oblique. The upward and 

 outward movement is produced chiefly by the superior and external recti, the infe- 

 rior oblique opposing the intorsion of the superior rectus. Motion downward and 

 outward is due to the external and inferior recti , the superior oblique opposing the 

 outward wheel action of the inferior rectus. The downward and inward motion is 

 due to the internal and inferior recti, the superior oblique opposing the inferior 

 rectus. 



When one muscle is weaker or larger than its opposing muscle, the eye is turned 

 to the side of the stronger, producing strabismus or squint. It is usually turned 

 laterally, most frequently to the inner side producing internal or convergent strabis- 

 mus. All the recti except the external are supplied by the oculomotor nerve. If 

 that nerve is paralyzed only the external rectus can act, and an external squint will 

 result. If the sixth cranial nerve (abducens) which supplies the external rectus is 

 paralyzed, the eye will turn inward, the superior and inferior recti opposing each 

 other. 



Paralysis of one or more muscles may occur. If a single muscle is involved it 

 is usually the superior oblique or external rectus, as each of these is supplied by a 

 separate cranial nerve, the fourth and sixth respectively. 



Although the third or oculomotor has a much wider distribution than these, sup- 

 plying all the other extrinsic muscles, as well as the ciliary muscle and sphincter of 

 the iris, when it is completely paralyzed the clinical picture is definite. Ptosis is 

 present and is due to paralysis of the levator palpebrae. External strabismus and 

 slight depression of the eye are produced by the unopposed action of the external 

 rectus and superior oblique, while the eye is otherwise motionless. The pupil is 

 dilated from paralysis of the sphincter of the iris, and accommodation for near objects 

 is lost from paralysis of the ciliary muscle. Slight exophthalmos appears from paral- 

 ysis of all but one of the recti muscles. 



The fourth nerve alone is rarely paralyzed. There will be little disturbance 

 of function, since the motion of the superior oblique is performed partly by tin- 

 other muscles. The eye will turn inward when the object looked at is lowered, 

 and upward only when the object is turned far toward the healthy side. One eye 

 must be closed to prevent double vision or diplopia. 



Of the single paralyses, that of the sixth nerve is most frequent on account of 

 its extended course from its origin in the brain to its peripheral termination in the 

 external rectus, rendering it liable to involvement by adjacent pathological proo 

 as meningitis, tumors, or hemorrhages. Such lesions may involve it alone, or 

 together with a series of cerebral nerves, paralyzed one after another from a progress- 

 ing pathological condition, which would then probably be at their central origin, or 

 in the \vall of the cavernous sinus, where they are close together. The sixth nerve 

 may be paralyzed by a fracture of the base of the cranium in the middle fossa. 



When the ophthalmic division of tin- fifth nerve is paraly/ed, there follows 

 anesthesia of the conjunctiva of the globe and upper lid, and of the other parts supplied 

 by this nerve. The lids do not respond reflexly, as usual, for protection of the 

 cornea, which is liable then to troublesome ulceration. 



The cervical sympathetic supplies the dilatator muscle of the iris, and reaches the 

 cranium along the internal carotid artery. When the cervical sympathetic is paralyzed 



