198 PHYSIOLOGY 



of the body, or of the afferent nerve passing from the surface to the 

 central nervous 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 

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



Sensory l\|i Sensory nerve 9 j 



Surface 



i 1 Central Nervous 



System 



tl 



FIG. 33. Diagram of a reflex arc. 



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 intestine, 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 



