Chap. 12 



CIRCULATION AND TRANSPORTATION BODY FLUIDS 



219 



Fig. 12.18. Network of blood vessels in the web of a frog's foot, darker ones 

 with venous and paler with arterial blood; a, arterioles; v, venules; x, a direct con- 

 nection between arteriole and venule. Many pigmented cells are scattered along the 

 capillaries. A view of the circulating blood in such a network can be easily ob- 

 tained by extending the moist web of a lightly anesthetized frog on the stage of 

 the microscope. (Courtesy, Maximow and Bloom: Textbook of Histology, ed. 6. 

 Philadelphia, W. B. Saunders Co., 1952.) 



in parts or the whole of the body. Cold or heat, food or lack of food, work or 

 rest, and turns of mind, such as mirth, fear, and worry are all significant in- 

 fluences, and worry is the most devastating in its effects. Sensory nerve fibers 

 in the arteries, especially in the arch of the aorta, also contribute to the rate 

 of the heartbeat. 



Stimulation of the paired inhibitor nerves (parasympathetic branches of the 

 vagus) slows the heartbeat. The impulses come from the vagus centers in the 

 medulla and pass over the nerve fibers into the walls of the heart, in and near 

 the pacemaker nodes (Fig. 12.19). Stimulation of the paired accelerator 

 nerves (sympathetic) quickens the heartbeat. Both nerves produce hormone- 

 like substances. Acetylcholine from the vagus nerves slows the action of heart 

 muscle and an adrenalinlike substance from the accelerator nerves quickens it. 

 The inhibitor and the accelerator nerves have been called the reins and the 



