27 6 
DR. U. PRITCHARD ON THE COCHLEA 
The last and most important difference is the presence of this lagena at the end of 
the membranous labyrinth (ductus) of the Monotreme’s cochlea. Nothing approaching 
it has ever been found in the spiral Mammalian cochlea, although a very similar con¬ 
dition is found at the apex of the Bird’s, as will be described later on. This little 
addition in the Duckbill forms another of the many links between the Mammal and 
Bird found in this peculiar animal. 
A brief sketch of the Bird's cochlea will now be necessary before any satisfactory 
comparison can be made, and it must be borne in mind that the cochlea of the Amphibia 
and Beptilia is similar in type, though less highly developed, to that of the Bird. 
As already stated, the bony labyrinth of the Bird’s cochlea is a slightly curved 
tube with an enlarged anterior extremity; within this is contained another more or 
less complete framework of dense connective tissue, cartilaginous in consistence, and 
very similar in structure to the so-called cartilage of the eyelid. 
At the vestibular extremity of the tube this cartilaginous framework consists of two 
pieces, one on each side of the tube, named, from their shape, the quadrilateral and 
triangular cartilages (see Plate 46, fig. 8). 
Stretching across the cochlea and joining these two is a strong membrane (membrana 
basilaris), and thus the tube is divided into two, which for convenience of description 
and clearness of comparison may be called the scala tympani and the ductus cochleae. 
These are not of equal size in any part of the cochlea tube; the upper or ductus 
cochleae being the larger even at the commencement, and the lower or scala tympani 
becoming smaller and smaller until it quite disappears by ending in a cul-de-sac before 
the enlarged anterior extremity, the lagena, is reached. 
On tracing the two cartilages forwards their triangular and quadrilateral forms 
become, first, more marked and then they increase in size. Not only do they enlarge 
generally, but their upper and lower margins extend further and further round the 
circumference of the tube until they meet both above and below; thus forming a 
complete cartilaginous tube, which is somewhat loosely adherent to the bony wall by 
means of connective tissue. 
As this cartilaginous tube is being completed, the scala tympani is gradually 
encroached upon, until it is entirely filled up by the cartilage, which at length also 
displaces the membrana basilaris ; thus there is only a single scala at the anterior end 
of the cochlea. Before the disappearance of the scala tympani the quadrilateral and 
triangular plates begin to diminish in thickness and gradually lose their characteristic 
forms, until in the lagena the cartilaginous tube becomes of equal thickness all round. 
The roof of the ductus is membranous; and this membranous roof is not a simple 
arch, but is re-duplicated into a number of longitudinal (to the tube) folds of some 
thickness, which in many Birds fill up the greater part of the interior of the ductus 
* This description has been taken from my own observations, which confirm most of the statements of 
Hasse and Meter. 
