NERVOUS SYSTEM AND BRAIN. 386 NERVOUS SYSTEM AND BRAIN. inward, unite to I'oiin the optic commisi?ure. the pons, and, passing along Kroni the coumiissuie two large bundles of fibres cavernus sinus, enters the — the optic tracts — pass backward and outward sphenoidal fissure. It su to the brain. The eye differs from all other oblique muscle. sense organs in that its nervous apparatus is develoiied by a direct outgrowth from the embry- onic neural canal. Later, the ingrowth of meso- blastic tissue separates the external or retinal part from the central nervous -system. This retinal part is analogous to the posterior root gangli(m of the spinal nerves, and, as is the case with the latter, its subsequent connection with the central nervous system is made by the growth of axones from the peripherally situated cells back into the central system, i'or structure of retina, see Eye. The third, or oculomotor nerve, is a purely motor nerve. Its fibres are the axones of a Outlimrds. Mem,lun.exU Nuc.v.c.e^l— li!- IxtretU. { Int.gran.l~-
>, .s V , (-^ — Z.ext.ffran.L
-yucJifiilr.fih. Ini.rettl: Gangn' Mar.lini TBANSVKi; L ■JfCilftb. mdjfiiirib. LTINA OF MAN. the outer wall of the orl)it through the pplies the superior Rds.&.cos.l. Vc.ext.nuc.l. £xtpl&x.l. Mp.cintjiucl. Iniplex.l. Gang.cel.1. yc.mi. SCHEME OF THE STRUCTPRE OF THE RETINA. MAN. Rds. & COS. I., rods and cones layer; V. c. ext. nuc. /., bodies of visual cells (externa! nuelear la,ver); Ex. ptrx. /., external plexiform layer; Gang. eel. /.. f?anKliou cell layer; JV'e. tib. I., nerve ftl)rc layer ; 1. rods ; 2. cones ; 3, cone gran- ule and fibre: 4. rod grannie and fibre; 5, bi-polar (ri>d) cells ; )!, bi-polar (cone) (-ells : 7. ganglion cells branching in different layers of the internal plexiform zone; 8. louer branching of bi-polar (rod) cells; 9, lower branching of bi- polar (cone) cells; 10, centrifugal nerve fibre; 11, Miiller's cells; 1*2, contact between the rods and the bi-polar (rod) cells; 1-3, contact between the cones and the bi-polar (cnnei cells; Hi. p. c. int. nuc. /., layer of bi-polar cells (internal nuclear layer): Int. plex. 1., internal plexiform layer. The fifth, trifacial, or trigemintis, is the largest of the cranial nerves. It resembles the spinal nerves in being composed of a motor part and a sensory part. The fibres of the smaller Inf.troch.nz. Pig. epitfi., pigment epithelium : Wrf.t. d' cna., rods and cones; Alem. /im. cxt.^ membrane Hmitans externa; .Vnc. V. c. e. //. /., nuclei of visual cells (external granular layer); £xt. reti. /., external reticular layer ; Jut. t/ran. /., internal granular layer; /»?. n^i. /., internal reticular layer; Ofitir/. eel. I., ganglion cells laver; Ne. fib. /., nerve fibres layer; Mar. lim. l^^, miirgo llmitans internus; Out. lim. rils., outer limbs of rnds^ In. tim. rtta., inner liinl)s of roils; Cos., cones; Z. txt. t/rati /,, zone of external granular layer free from nuclei (llenlis external fibre layer); Cd. gaiiq. ret.t cells of the g.inglion retina ; S'uc. .fHl. r. ,ritt., nuclei of ;ruller*3 radial fibres; .S>o«j/., SiHingfblasts; Milt, fill., .Miiller's fibres; Wed. Mill, fib., wedge of MUller's fibres. group of cells situated just to one side of the median line in the fioor of the iter beneath the corpora quadrigemina. Passing internally to the red nucleus, the nerve reaches the ventral surface of the mid-brain at the inner margin of the cere- bral peduncle just in front of the pons. The nerve runs forw;ird along the inner wall of the cavernus sinus, and, dividing into two parts, en- ters the orbit through the sphenoiilal fis>ure. The su|)<>rior branch of the nerve supplies the superior rectus and levator palpi'brir. The inferior branch ,su))plies the internal rectus, the inferior rectus, and the inferior oblique. The fourth, or pathetic nerve, is the smallest cranial nerve. Like the preceding, it is n purely motor ner"p. Its fibres originate in a group of rolls in the floor of the iter jtist lx>low the nucleus of the third nerve. The nerve winds around the outer side of the crus cerebri just in front of >-,_r -'Sttp.orb.ne. ||v -lajch.gl. ¥: -EUvpaL -fron.ne. fZ —£xL rcctm. Gasscr.gano. NERVES OF TIIF ORIIIT. SEEN FHOM ABOVE. Sup. troch. ?t^., suprfttrochlear nerve; /"/. frock. «c., in- fratrochleiir nerve; Sup. ohl. ??i., HUiierior "oblique muscle; Crib, pi, ith., cribriform plate of etnmoiil ; Sas. tic, nanal nerve; Op. ne., optic nerve; .ir<l ne.^ tliinl nerve; ith nc, faurtli nerve; Mot. rt., motor root; Setts, rt., sensory root; Slip. orb. »»•., fuipra-orltitnt nerve; TmcH. f//., lacli- rynial giantl ; Elev. jmL, elevator palpebne ; /■"'roH. «*•., frontal nerve; Jj-ich. «<■., lachrymal nerve; Ext. rect. m., external rerdis muscle; '>/>A., ophtluilmic ; Gasser. gang^ liasserian ifanelioii ; 'th ui. tiUU nerve; liec. Jil. <i. m., recurroiit tllameiil to dura matiT. motor root come from colls situated in the iiu-thilla. The fibres of the sensory root come fnun cells in the Gasserian pan^rlion. The cen- tral processes of these cells enter the pons and pass to two nuclei in the floor of the fourth ven-
