522 THE CRANIAL NERVES. 



nerves entirely in a crossed direction. There is no question that these 

 animals after destruction of the tubercula quadrigemina on one side are 

 mainly blinded in the opposite eye, since they use exclusively the eye 

 of the wounded side to guide them in their motions. 



In man, the partial blindness of both eyes, sometimes observed in 

 cases of hemiplegia, makes it probable that the transmission of sight 

 takes place by both the crossed and direct fibres of the optic tracts. 



The reflex influence which causes contraction of the pupil is also 

 transmitted, in the lower animals, in a crossed direction ; that is, the 

 stimulus of light falling upon the retina of one eye passes to the optic 

 tubercle of the opposite side. But here, owing to the transverse con- 

 nections between the central parts of the brain, the stimulus becomes 

 duplicated, and contractions are produced in the pupils of both eyes 

 simultaneously. This is because, although the sensitive impression is 

 conveyed inward to the nervous centres by one optic nerve only, when 

 transformed into a motor impulse it may be sent outward by both 

 oeulomotorrus nerves at the same time. Consequently if, in a pigeon, 

 one eye be blinded by removal of the opposite optic tubercle, both pupils 

 will still contract under the stimulus of light applied to the sound eye. 

 In examining one eye, therefore, either in animals or in man, to ascer- 

 tain whether or not its retina be sensitive to light, the opposite eye 

 should always be covered, in order to prevent its exciting a movement 

 by reflex action. 



Third Pair. The Oculomotorius. 



The oculomotorius nerve, so called because it supplies four out of six 

 of the muscles moving the eyeball, originates from a collection of gray 

 substance situated next the median line, beneath the tubercula quadri- 

 gemina and the aqueduct of Sylvius. As this group of nerve cells is 

 continuous with that which gives origin to the fourth nerve or pathe- 

 ticus, it is sometimes designated as the common nucleus of the oculo- 

 motorius and patheticus nerves. From this nucleus the fibres of the 

 oculomotorius nerve pass downward and forward, through the substance 

 of the crus cerebri, until they emerge, in the form of several flattened 

 bundles, from its inner border, a little in front of the anterior edge of 

 the pons Yarolii. From this point, the apparent origin of the nerve, 

 its fibres unite into a rounded cord, which runs forward and outward, 

 to penetrate the cavity of the orbit by the sphenoidal fissure. During 

 its transit along the walls of the cavernous sinus, it receives one or two 

 fine twigs of sensitive fibres from the trigeminus nerve. In entering 

 the orbit, it divides into several branches, and supplies, the superior, 

 inferior, and internal straight muscles of the eyeball, the inferior oblique, 

 and the levator palpebroe superioris. The oculomotorius is accordingly 

 concerned both in the vertical and lateral movements of the eyeball, and 

 in those of rotation; while of the two other muscular nerves of this 

 organ, the abducens and patheticus, one is connected only with the 

 movement of lateral abduction, the other only with that of rotation. 



