1908.] Structure and Relationships of the Labyrinth. 509 
kangaroo. It is interesting to note that the oval window is situated further 
back relative to the saccule and the cochlea than in birds or mammals. Its 
posterior margin is almost in contact with the ampulla of the posterior 
canal, 
The cochlea, ¢., is slightly constricted after leaving the lower and inner 
portion of the saccule, and in the neck formed by this constriction lies the 
aqueduct of the perilymph, dp. Passing downwards and inwards, the 
cochlea first bulges slightly and then tapers gradually to end as a cone. It. 
is interesting to notice that, at the tip, the cochlea ends with a little nipple- 
shaped projection, exactly similar to that found in some of the ratite birds, 
such as the emu, ostrich and rhea. 
The cartilaginous framework within the cochlea which supports the basilar 
membrane and the lagena, /., is in the form of a long ellipse, the major axis 
of which is, of course, parallel with the long axis of the cochlea. It does not 
appear to be rotated upon itself to any very noticeable extent, the two limbs. 
of the framework remaining anterior and posterior respectively throughout 
their whole course. In this respect the shape of the framework differs from 
that of birds; but it is similar in another respect to that of at least most 
birds in that, at its vestibular end, the cartilage bends sharply upon itself. 
The ductus perilymphaticus, or aqueduct of the perilymph, dp., may be 
demonstrated very clearly in the labyrinth of the monitor, and shows very 
interesting relationships. It arises on the outer surface of the labyrinth at. 
the junction of the cochlea with the saccule as a fairly wide tube. From this 
point it curves forward, then inwards, and finally backwards, being in contact 
with and encircling the neck of the cochlea, and reaches the inner posterior 
portion of that neck. The ductus perilymphaticus was traced by Retzius* 
to this point in an allied species (Psammosaurus caspicus), but he was unable 
to trace it further. Its further course is interesting, especially in view of the 
fact that it throws considerable light upon the relationship of the round 
window and perilymph recess to the cochlea as found in birds and mammals. 
After reaching the inner posterior surface of the cochlea, the aqueduct of the 
perilymph turns slightly downwards, and immediately widens out into a large 
cavity, roughly heart-shaped, and termed the perilymph recess, recessus peri- 
lymphaticus, or recessus scale tympani, 7.p. 
The perilymph recess has roughly five surfaces. The first of these surfaces 
is the uppermost, and is small, consisting merely of an opening o.p., by means 
of which the perilymph recess communicates directly with the arachnoid 
space, and it therefore permits of free interchange between the cerebro-spinal 
fluid and the perilymph. The inner wall passes from above and within 
* Retzius, ‘Das Gehororgan d. Wirbelthiere,’ vol. 2, p. 98. 
