524 Dr. A. A..Gray. On the Anatomical - [June 1, 
The next stage need only be mentioned, for it consists merely in the 
reduction of the sharp cone-shaped bulging to a slight round bulging, and is 
found in many mammals, such as the carnivora and ungulates.* 
The last stage is that found in the primates, including man. In it the 
bulging on the floor of the scala tympani has quite disappeared, its walls 
having been gathered in, as it were, until they are flush with the rest of the 
floor of the scala tympani. With this gathering-in process the round window 
must of necessity come to occupy a position in the outer wall of the scala 
tympani, and similarly the aqueduct of the perilymph comes to open out 
quite abruptly from the floor of the scala tympani. In the primates, the 
aqueduct of the cochlea is almost capillary in calibre throughout the earlier 
part of its course.t 
When the labyrinths of reptiles, birds, and mammals are investigated in 
this way, it is not difficult to see why misconceptions should have arisen as 
to which structures in one division correspond with the structures in the other 
divisions. Thus, what is known as the ductus perilymphaticus in the reptile 
does not exist in the bird and mammal so far as present investigations show, 
and, conversely, the ductus perilymphaticus of mammals and birds is repre- 
sented merely by an oval dehiscence in the roof of the perilymph recess of 
reptiles. 
Again, the round window of man and the primates, with most other 
mammals, corresponds really to two structures in echidna, birds and 
reptiles, these two structures having come to coincide in the primates, etc. 
From the point of view of evolution the round window of man and most 
mammals corresponds with the opening low down in the perilymph recess of 
birds and reptiles, which is closed bya membrane. But as regards its position 
relative to the cochlea, the round window of man and mammals corresponds 
with the oval opening which exists between the cochlea and the perilymph 
recess of the echidna, the birds, and some reptiles. 
Further, by tracing the labyrinth through its changes in reptiles, birds, and 
mammals by the means of investigation employed, the process by which the 
ductus perilymphaticus comes to open out of the cochlea is made clear. 
For it is to be noted that this aqueduct is found in the amphibians before 
the cochlea has made any appearance as a separate cavity, it being repre- 
sented only by the macula basilaris. In this division, of course, the ductus 
perilymphaticus opens out of the vestibule. 
It is somewhat difficult to describe these changes in words, and a series of 
diagrams has therefore been drawn up to show the matter more clearly. 
* Gray, op. cit., vol. 1, pp. 9; Tez, ete: 
+ Gray, op. cit., vol. 1, p. 33. 
