408 VERTEBRATE LIFE AND ORGANIZATION 



is closed much ot the time, and the throat movements then simply move 

 air in and out ot the mouth and pharynx where gas exchange also occurs. 



Voice. A mechanism for sound production is closely associated 

 with the respiratory system. Two longitudinal, elastic bands, the vocal 

 cords, are situated in the laryngotracheal chamber near the glottis (Fig. 

 21.11). Air forced from the lungs sets the free edges of these cords in 

 vibration, and they in turn vibrate the column of air in the pharynx 

 and mouth. The pitch of the sound is controlled by muscular tension 

 on the vocal cords. Some of the expelled air inflates the vocal sacs, 

 which serve as resonating sacs and considerably increase the volume of 

 the sound. The vocal sacs may be paired evaginations from the lateral 

 walls of the pharynx, or there may be a single median vocal sac ventral 

 to the floor of the pharynx. Contraction of muscle fibers in the wall of 

 the vocal sac returns the air to the lungs, and the same air can be used 

 repeatedly. Some frogs, such as the bullfrog, can even call from beneath 

 water. 



The vocal cords are more prominent in males than in females, and 

 only the males have vocal sacs. The males gather first in the breeding 

 ponds during the spring, and their familiar croaking attracts the females 

 of the appropriate species. The females recognize the voice of the 

 males of their own species and come to them. 



187. Circulatory System 



The circulatory system is the transport system of the body. It con- 

 sists of the circulating fluids, chiefly blood, and of the heart and a series 

 of vessels that carry the fluids. As explained in Chapter 3, blood is com- 

 posed of a liquid plasma, in which red cells, white cells and throm- 

 bocytes are suspended. The thrombocytes are spindle-shaped cells con- 

 cerned with blood clotting. The exchange of materials between the 

 blood and the tissues occurs in the microscopic, thin-walled capillaries 

 situated between the arteries and veins. Food, oxygen and water leave 

 the capillaries, and carbon dioxide and other wastes enter them to be 

 removed by the veins. A volume of water nearly equal to the amount 

 that left the capillaries also reenters them. Some liquid remains in the 

 tissues and is returned by lymph vessels, which usually parallel the veins 

 and eventually empty into them. Before connecting with the veins, some 

 of these vessels lead into lymph sacs. Unusually large lymph sacs lie 

 ventral to the vertebral column and beneath the skin, separating it from 

 most of the underlying musculature (Fig. 21.8). 



Arteries. The pattern of the major blood vessels of the frog is 

 shown in Figure 21.12. Many, though not all, of these vessels are also 

 present in the higher vertebrates, including man. A pair of arteries, each 

 known as the truncus arteriosus, leave the front of the heart. Each soon 

 divides into three vessels— carotid arch, aortic arch and pulmocutaneous 

 arch. Each carotid arch extends anteriorly and divides into an external 

 carotid supplying the tongue and adjacent parts, and an internal carotid 

 supplying the upper parts of the head and the brain. A swelling at the 

 base of the internal carotid, the carotid gland, is believed to equalize 



