596 AMPHIBIA, 
The various valves and the conditions of pressure are such that the 
venous blood passes by the pulmonary artery to the lungs, the next 
quantum of blood enters the systemic arches, and the nearly pure 
arterial blood from the left side of the ventricle passes into the carotids. 
To understand the mechanism, it is necessary to consult some book 
with a complete anatomical decenipten, especially Gaupp’s edition of 
Ecker and Wiedersheim’s Anatomie des Frosches (1899). 
Spleen, thyroid, and thymus.—The spleen is a small red 
organ lying in the mesentery near the beginning of the large intestine. 
The thyroid is represented by two little bodies near the roots of the 
aortic arches. The thymus, perhaps originally associated with the 
gill-clefts, lies on each side just behind the angle of the lower jaw. 
Respiratory system.—The larval frog breathes at first 
through its skin, then by gills. The adult frog breathes 
chiefly by its lungs, but some cutaneous respiration is still 
retained, for even without its lungs a frog may live for 
some time, and it does not use them when hibernating. 
The lungs arise as outgrowths of the cesophageal region 
of the gut, and are connected with the back of the mouth 
by a short laryngo-tracheal tube, whose slit-like aperture is 
the glottis. Each lung is a transparent oval sac, with 
muscle fibres in its walls. The cavity is lessened by the 
spongy nature of the internal walls, which form numerous 
little chambers bearing the fine branches of blood vessels. 
In respiration the mouth is kept shut, and air passes in 
and out through the nostrils. A frog will die of asphyxia 
if its mouth be artificially kept open for a considerable 
time. When the floor of the mouth is lowered, and the 
buccal cavity thus increased, air passes in. When the 
nostrils and the opening of the gullet are shut, and the 
floor of the mouth at the same time raised, air is forced 
through the glottis into the lungs. When the pressure on 
the lungs is relaxed, and when the muscles of the sides of 
the body contract, the air passes out. e 
Excretory system.—The paired kidneys are elongated 
organs situated dorsally and posteriorly beside the urostyle. 
The waste products which they filter out of the blood pass 
backward by two ureters which open separately on the 
dorsal wall of the cloaca, and are not directly connected 
with the bladder. The ureter or Wolffian duct is seen as 
a white line along the outer side of each kidney; in the 
male it functions also as the duct of the testis. On the 
ventral surface of each kidney is a longitudinal yellowish 
