OF THE ORNITHORHYNCHUS PLATYPUS. 
275 
But on proceeding up the spiral, the floor becomes broader and the height lessened, so 
that the acute angle formed by the membrane of Reissuer and the lamina spiralis 
becomes more and more acute. These changes in form are not nearly so marked in 
the Ornithorhynchus; the membrana basilaris does get broader and broader, but not 
nearly in the same degree as in the typical Mammal. As regards the peculiar changes 
in the organ of Corti, such as the lengthening of the rods, &c., which are so marked in 
the spiral cochlea, they are the same, but less marked in the Ornithorhynchus so far 
as I have been able to trace them out. 
In the membrane of Reissner of this Monotreme the epithelial cells are thicker 
than in the typical cochlea; but the great feature of difference is the presence of 
blood-vessels running across the membrane from lamina to ligament, forming here and 
there convoluted knots. These I have never found in any other Mammal, nor have 
they been described in this situation as far as I am aware. 
The ligamentum cochleae of the Duckbill, both as regards its shape and structure, 
scarcely differs at all from that of the typical Mammal; and the same may be said of 
the stria vascularis and its other epithelial lining. 
The lamina spiralis ossea, with its limbus and sulcus, present no special points of 
difference. The same may be said of the membrana basilaris, with this slight 
exception—that whereas in the ordinary Mammal one large blood-vessel (the vas 
spirale) runs through its lower stratum just below the rods of Corti, in the Duckbill 
two small vessels are generally to be found (see Plate 46, fig. 6)—one in the usual posi¬ 
tion, and the other a little further inwards (towards the ligament). 
The organ of Corti itself differs very slightly from that of the ordinary cochlea. 
There are the same number of rows of bristle cells as are found in the lower Mammals,* 
namely, one on the outer side of the rods and three on the inner. The trabeculae are 
thicker, and their feet larger than usual; so much so, that the lower part of the first 
trabeculum may be easily mistaken for the foot of the inner rod. 
The rods of Corti are somewhat different; they stand more upright, so that the 
tunnel is not so wide ; the extremities are not so well developed, the feet being smaller 
and the heads not so broad. I have not been aide to make even an approximate 
calculation of the number of rods or bristle cells, but there must be a much fewer 
number of these in the Duckbill’s cochlea than in that of the ordinary Mammal. 
The course taken by the cochlear nerve and its branches differ in no essential points 
from those of the typical Mammal. There is in the former a ganglion very similar in 
relative position and component cells to the ganglion spirale. The only differences are 
that, whereas in the spiral cochlea the nerve trunk necessarily runs at right angles to 
the lamina spiralis, in this cochlea it runs parallel to the corresponding lamina; and 
that in the Duckbill the terminal branches pass to supply a lagena, an organ not found 
in the ordinary Mammal. 
* In the primates there are five and six rows of bristle cells in the upper part of the spiral, as pointed 
out in my paper above cited. 
MDCCCLXXXI. 2 O 
