576 AMPHIBIA. 



Mechanism of the heart. We cannot do more than refer to 

 the difficult question, how it is that the blood is propelled from the 

 ventricle along the proper channels. The right half of the ventricle 

 being nearer the right auricle, contains more impure blood, and it is 

 from the right side of the ventricle that the truncus arteriosus arises. 



The middle of the ventricular cavity contains mixed blood. The 

 left corner contains pure blood received from the pulmonary veins. 

 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 \Viedersheim's Anatomic des Frosches (1899). 



Spleen, thyroid, and thymus. The spleen, which is probably, 

 as in some other animals, concerned with blood-making, is a small red 

 organ lying in the mesentery near the beginning of the large intestine. 

 The thyroid, which is believed to have something to do with maintain- 

 ing the health of the blood, 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 external gills, and, finally, by 

 internal 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 oesophageal 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. 



