Responses of Higher Animals: The Effectors - 445 



watch the pigment granules flow inward 

 from the many fine branches of the pigment 

 cell until they accumulate as a very small, 

 deeply situated, mass around the nucleus 

 (Fig. 24-13). Or by treating the scale with 

 other drugs, such as acetylcholine (p. 455), 

 one can watch the pigment granules dispers- 

 ing into the many finer branches of the cell 

 (Fig. 24-13). 



The protoplasmic branches of the pigment 

 cells were previously thought to be pseudo- 

 podia, capable of extension and retraction. 

 But now it is known that the branches are 

 relatively permanent, and that the pigment 

 granules ebb and flow through the same 

 channels with each successive "expansion" 

 and "contraction." Thus the pigmentary re- 

 sponse probably represents a type of proto- 

 plasmic streaming (p. 198) rather than a kind 

 of amoeboid movement (p. 196). 



In the intact animal, pigmentary responses 

 are protective in nature, since they provide 

 for changes in coloration in accordance with 

 changes in the background of the environ- 



ment (Fig. 24-14). If a fish, for example, is 

 placed in an illuminated aquarium with a 

 white, or light-colored bottom, the skin of 

 the fish blanches as a result of a regimented 

 contraction of all the melanophores. But if 

 the bottom of the aquarium is changed to 

 black, the skin rapidly darkens, and all the 

 melanophores are found to be expanded. 



The normal stimulus that initiates the 

 responses of the pigmentary effectors is light 

 acting through the medium of the eyes and 

 nervous system. Thus a blind fish usually re- 

 mains permanently dark in contrast to the 

 changing shades of its fellows in the aquar- 

 ium. Among amphibians and crustaceans, 

 however, excitations from the eyes are not 

 transmitted to the pigmentary effectors; 

 rather they act upon the endocrine system. 

 In these animals, therefore, the immediate 

 response is elicited by hormones. And even 

 among fish and reptiles, the chromatophores 

 are partially controlled by hormones, al- 

 though excitations from the nervous system 

 play a more dominant role (p. 454). 



TEST QUESTIONS 



Name five kinds of effector organs found in 

 fish and other lower vertebrates. Which kinds 

 are also found in man? 



Distinguish the three kinds of muscle tissue 

 on the basis of (a) structural differences and 

 (b) functional differences. 



What common structural and functional 9. 

 features are found in all muscle, regardless 

 of type? 



Specify three functions that could not be 10. 

 achieved unless the muscles were arranged in 

 antagonistic groups. 



Distinguish between a single twitch and a 

 tetanus. 



Draw a curve to simulate the single twitch of 

 a frog's muscle, designating the name and 11. 

 duration of each part of the curve. 

 Explain the relationship between: (a) the 

 duration of the recovery period and the 12. 

 susceptibility of a muscle to fatigue; (b) 

 graded contractions and the all-or-none law. 13. 

 When a working muscle is deprived of oxy- 

 gen: (a) lactic acid accumulates more rapidly; 



(b) the glycogen reserves are used up faster; 



(c) inorganic phosphates accumulate; and (d) 

 the muscle becomes fatigued more quickly 

 and loses its capacity to contract. Explain 

 these facts in the given order, emphasizing 

 the relationships between them. 



How is it possible for a muscle to do work 

 even though it may have no available oxy- 

 gen and no capacity to hydrolyze glycogen? 

 Explain how and why a steadily working 

 muscle accumulates an oxygen debt. How is 

 the extra oxygen used when a muscle is al- 

 lowed to rest, and how is this usage related 

 to a partial restoration of the glycogen re- 

 serves of the muscle? 



Differentiate between myosin and actomysin. 

 Explain the dual role of myosin in relation 

 to the contractile process. 

 Discuss the efficiency of a muscle as compared 

 to other machines. 



What use to the body is subserved by the 

 energy that escapes conversion into mechan- 

 ical power during muscular work? 



