CHORDATA. 



upper utriculus and a lower sacculus, the connecting utriculo-saccular duct 

 being narrow in the mammals. Both utriculus and sacculus receive a 

 part of the macula acustica. Outgrowths from the vesicle occur, giving 

 the whole the name of labyrinth. From the utriculus arise three semi- 

 circular 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 three 

 dimensions of space and without doubt subserve the function of equili- 



S3 2 - FlG - 533- 



FIG. 532. Membranous labyrinths of (7) Myxine; (II) Chimara (after Retzius). 

 a, ampulla;; c, undivided vesicle with endolymph duct; dp, endolymph duct;/, anlage of 

 lagena; n, macula neglecta; 5, sacculus; su, sinus utriculi; U, utriculus; i, anterior, 2, 

 posterior, 3, outer semicircular canals. Auditory nerve and its branches dark. 



FIG. 533.- Human ear (based on Schwalbe). a, incus and stapes; ae, external 

 meatus; c, cartilage; de, endolymph duct; //, malleus; K', temporal bone; K/, bony 

 labyrinth; Na, nerve; o, base of conch; ,<;, sacculus; T, tympanic membrane; /, fenestra 

 rotunda; te, Eustachian tube; u, utriculus. 



bration (p. 117). They are an outer horizontal, an anterior vertical 

 (nearly sagittal), and a posterior vertical (nearly transverse). The non- 

 ampullar ends of the two vertical canals unite, a condition which is under- 

 stood when it is recalled that in cyclostomes these canals alone are present, 

 and in Myxine (fig. 532, 7) form a single canal with two ampulke. A late 

 formation is an outgrowth from the sacculus, which appears even in the 

 fishes as a small pocket, the lagena, containing a part of the macula 

 acustica; in the reptiles and birds the lagena becomes much larger, and 

 in the mammals is spirally coiled and is known as the cochlea. A part of 

 the macula acustica of the lagena develops into a special nerve-end appa- 

 ratus, the organ ofCorti. 



