iS 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 capillaries 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 Lepidosiren ; 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 Siren." 



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. 



