512 TEXTBOOK OF ZOOLOGY 



are therefore enclosed in one large vessel for a short distance before 

 breaking up into three separate vessels. The carotid arch originates 

 from the anterior compartment, the systemic arch from the middle 

 compartment, and the pulmocutaneous arch from the posterior com- 

 partment. Blood enters the anterior and middle compartments from 

 the synangium, but enters the posterior compartment, or pulmocuta- 

 neous arch, from the pylangium. 



The heart beats in a wavelike peristaltic manner. The sinus venosus 

 contracts first, then the auricles (the right auricle preceding the left 

 by a moment), then the ventricle, and finally the conus. 



Venous blood from the right auricle enters the right side of the 

 ventricle, and oxygenated blood from the left auricle enters the left 

 side. Muscular ridges of the ventricular wall tend to hold the blood 

 and reduce mixing. Since the heart's contractions are wavelike, 

 the ventricle immediately forces the blood into the conus through 

 the semilunar valve. Venous blood from the right auricle is closest 

 to the conus, and it passes out first, flowing into the closest open- 

 ing offering the least resistance. This is the opening in the 

 pylangium to the pulmonary arch, leading to the lungs. As the 

 contraction of the ventricle comes to an end, forcing out the re- 

 maining oxygenated blood, the pylangial part of the conus contracts, 

 bringing the spiral valve against its ventral wall. This action, 

 together with that of the synangial valves which are anterior to 

 the common opening of the pulmonary arches, completely shuts off 

 the flow of blood into these arches. The blood therefore passes into 

 the synangium and enters the chambers leading to the systemic 

 arteries or the carotid arteries. Since the carotid arteries offer 

 some resistance to blood flow, the blood tends to enter the larger 

 systemic arteries first. As the systemic arteries fill, they offer more 

 resistance to the blood, while resistance in the carotid arteries de- 

 creases due to their emptying into capillaries ; so the last oxygenated 

 blood from the ventricle passes into the carotids and is conveyed 

 to the head region. 



The heart must beat sufficiently fast and pump a sufficient volume 

 of blood at each stroke to insure an adequate supply of oxygen and 

 food to the body tissues, as well as to remove waste products as 

 they form. The rate of pulsation is influenced greatly by tempera- 

 ture up to a certain maximum rate, for the activity and metabolism 

 of the bullfrog are considerably affected by temperature. Blood 



