RESPONSIVENESS IN VERTEBRATES 



innervated but also in the medulla of the adrenal gland. These medullarv 

 cells then release epinephrine and norepinephrine into the blood stream; 

 thus, additional amounts of these substances reach responsive efTectors all 

 over the body- For this reason, the response to sympathetic discharge is 

 more widespread than that to parasympathetic discharge. 



Certain organs supplied by the autonomic nerves receive both sympathetic 

 and parasympathetic fibers and respond in opposite fashion to the continuous 

 flow of impulses from the two sets of fibers (Fig. 4.23). This dual innervation, 

 and the antagonistic responses invoked, constitute the nervous mechanism for 

 subconscious regulation of the activities of vital organs. The response of 

 the organ is determined by the balance in intensity, so to speak, of the 

 opposing stimuli. In general, the reactions to sympathetic outflow, aided by 

 the adrenal medulla, enable the individual to mobilize latent resources in the 

 event of emergencies. Responses to parasympathetic outflow, on the other 

 hand, tend to conserve or restore resources. For example, in the regulation 

 of the rate of heart beat, sympathetic discharge tends to increase the rate, 

 parasympathetic, to slow it down. During an emergency, when increased 

 flow of blood is advantageous, sympathetic discharge predominates over para- 

 sympathetic and the heart beats faster. When the emergency is over, the 

 parasympathetic outflow predominates and the heart beat slows to its normal 

 rate. 



Regulation by Endocrines. The endocrines, hormones, or internal 

 secretions are the products of the ductless or endocrine glands (Fig. 4.8). 

 Endocrines can produce coordination of a type comparable to that produced 

 by a simple reflex arc, although more often the regulatory effect is widespread. 

 Chemical coordination by means of circulating hormones was discovered by 

 Bayliss and Starling in 1902 in connection with the flow of pancreatic juice 

 mto the small intestine. This occurs only when food enters the duodenum 

 from the stomach. It was supposed for a long time that the release of 

 pancreatic juice was conditioned by nervous coordination. Experiments dem- 

 onstrated that the nerves leading to the pancreas could be cut without 

 affecting the control of the flow of the pancreatic juice when food entered 

 the small intestine. It was then discovered that if the blood vessels are tied 

 so that blood does not enter the capillaries of the pancreas, the juice is 

 not released. Further study revealed that, when the contents of the stomach 

 enter the small intestine, the hydrochloric acid stimulates certain cells in the 

 lining of the duodenum to discharge into the blood an endocrine substance 

 known as secretin. Secretin, reaching the pancreas by way of the blood, 

 stimulates the acinar cells of the pancreas to release pancreatic juice, which 

 then passes to the intestine by way of the pancreatic duct. In a similar way, 

 the release of bile from the gall bladder is conditioned by the secretion of 

 cholecystokinin from cells in the intestinal lining when stimulated by the aciditv 

 of the food mass entering from the stomach. Cholecystokinin is carried to 

 the gall bladder by the blood. 



Hormones, like vitamins, are substances required in verv small amounts to 



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