542 



CHORD AT A. 



be added. It arises from the inner angle of the eye, and can extend over 

 tlie cornea beneath the upper and lower lids. A special laeliryraal gland, 

 which occurs at the outer angle of tlse eye, provides the fluid to moisten 

 the cornea, while a second or Harder's gland occurs at the inner angle 

 when a nictitating membrane is present. Both are lacking in tlie An- 

 aninia. 



The ear, at the level of the medulla oblongata, rivals the eye in 

 its complication of structure. In development it has one point in 



common with the invertebrate 

 otocyst — it arises as an ecto- 

 dermal pit which is usually 

 completely cut off from its par- 

 ent layer, and only in elasmo- 

 branchs remains connected with 

 the exterior by a tube, the 

 elsewhere closed endolymphatic 

 duct. In the cyclostomes it con- 

 sists of a single vesicle with a 

 single macula acustica; from 

 the fishes upwards the vesicle 

 becomes divided by a constric- 

 tion into an upper utriculus 

 and a lower sacculus (fig. 575). 

 the connecting utriculosaccular 



Fig. 575— Diagram of membranous laby- duct being narrow in the mani- 

 riDth of a fish. (From Wiedersheim.) i -r, j.i x • i i 



.(((, ae, ap, anterior, e.x;ternal, and poste- malS. BotJl utriculus and Sac- 

 rior ampullte ; a.ss-, superior utricular -t ' - 1 . . .f f tl 



sinus ; ca, re, cjj, anterior, external, and CUIUS receive a pai'T. 01 tlie 



posterior semicircular canals; t».s,utri- ,„„„,,i,, .Tr.iii+i.-..i Hi vovti.^nlo 



culosaccular canal; dc. ductus en- mSlCnla aCUstKa. UneitlCUla 



dolymphaticus ; (, lagena; ire, recessus f,.„,,, +],„ vp^iolp Ofonr o-ivino- 



uti-iculi ;.•.?, sacculus utriculi ; .<.,s, supe- HOm ^'^^' ^eslCie OCtUl, glMllg 



rior utricular sinus ; sp, posterior utri- +!,„ -.vhole tbo mmp of hibvriutll 



r:\ilar sinus: 11, utriculus; +, origin of en- ''"'' "JiOiC Lllc name Oi laoMiiuu. 



d.dymph duct. Pj.qjj^ tl^g utriculus arise three 



semicircular canals, connected at either end with this cavity, each 

 swollen at one end to an ampulla, containing a special nerve 

 termination, the crista acustica. These canals stand at right 

 angles to each other in the tliree dimensions of space and with- 

 out doubt subserve the sensation of e(|nilibration (p. 128). They 

 are an outer horizontal, an anterior vertical (nearly sagittal), and 

 a posterior vertical (nearly transverse). The nou-ampullar ends 

 of the two vertical canals unite, a condition which is understood 

 when it is recalled that in cyclostomes these canals alone are 

 present, and in A[yxine form a single canal with two ampulla?. 

 A later formation is a diverticulum from the sacculus, which 



