178 PHYSIOLOGY 



system. We can proceed further and cut the efferent nerve away from 

 the central nervous system and still succeed in exciting a condition of activity 

 in the efferent nerve or in its attached muscle. All parts of the reflex arc 

 possess the property of excitability, and we are thus able to arouse the 

 activity of each part in turn, to study its conditions, its time relations, 

 and the physical and chemical changes concomitant with the state of 

 activity. 



It will be convenient for our analysis to begin with the tissue whose 



IV ! 



: J Central Nervous 

 Sensory ll^v Sensory nerve 9 ! 



* nrr 



FIG. 33. Diagram of a reflex arc. 



reaction forms an end link in the reflex chain, namely, the muscle, and to 

 proceed from that to the consideration of the processes occurring in the 

 conducting strand between central nervous system and muscle, namely, 

 the nerve fibre, postponing to a future chapter the treatment of the more 

 complex processes associated with the central nervous system. 



In the higher animals we may distinguish several varieties of muscle. 

 All movements that require to be sharply and forcibly carried out are 

 effected by means of striated muscular tissue, and as these movements 

 are in nearly all cases under the control of the will the muscles are generally 

 spoken of as voluntary. Unstriated or involuntary muscles "from sheets 

 or closed tubes surrounding the hollow viscera By their sFow, prolonged 

 contractions they serve to maintain and regulate the flow of the content^ 

 of these organs. Such fibres are found surrounding the blood-vessels, 

 the alimentary canal, the bladder, &c. Intermediate in properties as 

 well as structure between these two classes is the heart muscle. This, 

 like voluntary muscle, is striated, but presents considerable variations both 

 in structure and function from ordinary skeletal muscle. Many of its 

 properties will be considered in treating of the physiology of the heart. 

 The properties of contractile tissues have been most fully investigated in the 

 voluntary muscles, almost exclusively on the muscles of cold-blooded 

 animals, such as the frog. The choice of skeletal muscles for this purpose 

 is justified by the fact that a function is most easily investigated in the 

 organs in which it is most highly developed. The choice of cold-blooded 

 animals is guided by the fact that it is possible to isolate the muscle from 

 the rest of the body and to study its reactions during a considerable time 



