59* 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 description, 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. 



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 



