226 
leading into it. In constant association with the gas gland is the 
rete mirabile duplex, a structure which is probably of as much 
importance for the filling of the bladder with oxygen as the gas gland 
itself. The rete mirabile consists of the intimate intermingling of the 
two sets of fine capillaries formed by the subdivision in the same 
region of the body of the artery and vein which supply the gas gland — 
the artery breaks up into a bunch of hundreds of fine capillaries 
carrying blood of course to the gland, and closely intermingled with 
these are the equally numerous and fine capillaries of the vein 
returning the blood from the gas gland. This rete mirabile may be 
bipolar, i. e., the arterial capillaries at the end of the rete next the 
GAS GLAND 
EPITHELIUM 
GAS GLAND 
EPITHELIUM 
RETE MIRABILE UNIPOLARE DUPLEX 
Fig. 1. 
gland may unite into a few large vessels before again subdividing to 
form the capillaries supplying the gas gland, the venous capillaries 
uniting in a corresponding manner (found in eels e. g.), or unipolar, 
the arterial capillaries of the rete directly supplying the gas gland 
cells, and the corresponding venous capillaries returning from the gland 
at once mingling with the arterial capillaries without first uniting into 
a few large veins. The essential feature of the rete mirabile is the 
intimate intermingling and juxtaposition (not inter- 
communication) of the two sets of capillaries carrying 
blood in opposite directions. ‘Thus in all deep-water teleost 
fishes which migrate vertically the bladder contains oxygen, usually 
in large quantity, and this is abstracted from the gland and pumped 
into the bladder by means of the mechanism — the oxygen gland 
and rete mirabile — just described. 
In addition to the gross structure of this mechanism, previous 
