8 REPTILES AND BIRDS. 
the circulation being aided by the contraction of the surrounding 
muscles.” 
The functions of gills are described by the Professor with great 
minuteness. ‘The main purpose of the gills of fishes,” he says, 
“being to expose the venous blood in this state of minute sub- 
division to streams of water, the branchial arteries rapidly divide 
and sub-divide until they resolve themselves into microscopic 
capillaries, constituting a network in one plane or layer, supported 
by an elastic plate, covered by a tesselated and non-ciliated epithe- 
lium. This covering and the tunics of the capillaries are so thin as 
to allow chemical interchange and decomposition to take place 
between the carbonated blood and the oxygenated water. The 
requisite extent of the respiratory field of capiuaries is gained by 
various modes of multiplying the surface within a limited space.” 
“Each pair of processes,” he adds, “has its flat side turned towards 
contiguous pairs, and the two processes of each pair stand edgeway 
-to each other, being commonly united for a greater or less extent 
from their base ; hence Cuvier describes each pair as a single bifur- 
cated plate, or ‘ feuillet.’” 
The modification which takes place in the respiratory and 
other organs in Reptilia is described in a few words. ‘“ Many 
fishes have a bladder of air between the digestive canal and the 
kidneys, which in some communicates with an air-duct and the 
gullet ; but its office is chiefly hydrostatic. When on the rise of 
structure this air-bladder begins to assume the vascular and 
pharyngeal relations with the form and cellular structure of lungs, 
the limbs acquire the character of feet: at first thread-like and 
many-jointed, as in the Lefzdosiren; then bifurcate, or two-fingered, 
with the elbow and wrist joints of land animals, as in Amphiuma ; 
next, three-fingered, as in Proteus ; or four-fingered, but reduced to the 
pectoral pair, as in Szven.” 
In all Reptiles the blood is conveyed from the ventricular part 
of the heart, really or apparently, by a single trunk. In Lepidosiren 
the veins from the lung-like air-bladders traverse the auricle which 
opens directly into the ventricle. In some the vein dilates before 
communicating with the ventricle into a small auricle, which is not 
outwardly distinct from the much larger auricle receiving the veins 
of the body. In Proteus the auricle system is incomplete. In 
Amphiuma the auricle is smaller and less fringed than in the Sirens, 
the ventricle being connected to the pericardium by the apex as well 
as the artery. ‘This forms a half spiral turn at its origin, and dilates 
into a broader and shorter bulb than in the Sirens. 
