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 x 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 



II 



Sensory \\K Sensory nerve 

 Surface\T 



r^ 



j 1 Centra/ Nervous 



System 



\\\\\\v 



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 form sheets 

 or closed tubes surrounding the hollow viscera. By their slow, prolonged 

 contractions they serve to maintain and regulate the flow of the contents 

 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 tissue's have been most fully investigated in the volun- 

 tary 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 without the research being interfered with by the death of the tissue. We may 

 therefore deal at length with the properties of the skeletal muscles, pointing out inci- 



