2 D’ARCY W. THOMPSON ON THE AUDITORY 
In the membranous labyrinth of Orthagoriscus the following parts are dis- 
tinguishable : Utriculus with Sinus superior; Recessus utriculi; the three semi- 
circular canals with their corresponding ampulle; and the Sacculus and Lagena 
hardly separated from one another or from the Utriculus. Six nerve-endings are 
visible, three cristee ampullarum, macula recessus utriculi, macule sacculi and 
lagen ; but I sought in vain for the macula neglecta. The Utriculus is a roundish 
sac, passing gradually in its upper part into the thin-walled conical Sinus superior, 
which terminates by bifureating into the anterior and posterior canals, without 
extending into any free “apex” between them. Anteriorly it opens into the wide 
recessus utriculi from which it is marked off by a scarcely notable constriction : at 
the base of the latter cavity is a well-marked and rather large macula recessus, 
opposite which is a little pouched diverticulum. The recessus is connected above 
with the ampula anterior and externally with the ampulla externa, which both open 
Fig. 3. 
Fig. 2, Right auditory labyrinth, inner side. Fig. 3. Right auditory labyrinth, outer side. 
U. utriculus, S. sacculus, LZ. lagena, rec. recessus utriculi; a.s.c., p.s.c., h.s.c., anterior, posterior, and horizontal 
semicircular canals; a.a., a.p., a.h., their ampulle ; s.s., sinus superior ; m.7., m.s., m.l., macula recessus, mac. sacculi, 
and mace. lagenz. 
somewhat widely into it. The ampulla posterior, rather more perfectly formed, 
opens by a narrow neck into the hinder part of the utriculus, just above the 
entrance into it of the horizontal canal. The sacculus and lagena 
form a single common cavity, and are only entitled to the retention 
of their names by still possessing their separate nerve-endings. ‘They 
are neither separated from one another nor from the utriculus, of 
which they simply form a slightly bulging expansion on its inner 
side.* 
Fig. 4. 
Fig. 4. Diagrammatic transverse section through vestibule. 
*In a smaller specimen which I had an opportunity of dissecting last July in M. De Lacazp 
Duruiers’ Laboratory at Roscoff, the sacculus was separated from the utriculus by a slightly more marked 
constriction. 
