454 TEXT-BOOK OF PHYSIOLOGY. 



the living animal the two portions will contract and separate them- 

 selves to a certain distance. This indicates that the muscle when in 

 a state of rest is in a slight degree of contraction. This condition of 

 the muscle, to which the term muscle tonus is given, was formerly 

 attributed to an automatic and continuous discharge of energy from 

 the nerve-cells, h Brondgeest, however, showed that this tonus is 

 entirely reflex in origin and immediately disappears on division of 

 the posterior roots of the spinal nerves, which would not be the case 

 if the cells in the cord were acting automatically!! The afferent 

 nerves in this reflex arise in the muscle or its tendons, and the stim- 

 ulus is the slight degree of extension to which the muscle is subjected 

 in virtue of its attachments and the ever- varying position of the limbs 

 and trunk. 



The tonic contraction of the visceral muscles, e. g., the pyloric, 

 the vesical, the anal sphincters, though regarded as automatic by 

 some, is probably reflex in origin, dependent on the arrival of 



FIG. 207. DIAGRAM OF A SIMPLE REFLEX ARC. i. Sentient surface. 2. Afferent 

 nerve. 3. Emissive or motor cell. 4. Efferent nerve. 5. Muscle. (After Moral 

 and Dayon.) 



afferent impulses from the periphery. It is probable that future 

 investigation will disclose the existence and pathway of these afferent 

 fibers. 



Reflex Actions. It has already been stated that the nerve-cells 

 in the spinal cord are capable of receiving and transforming afferent 

 nerve impulses into efferent nerve impulses, which are transmitted 

 outward to muscles, exciting contraction; to glands, provoking secre- 

 tion; to blood-vessels, changing their caliber; and to organs, inhibit- 

 ing or accelerating their activity. All such actions taking place 

 through the spinal cord and medulla oblongata independently of 

 sensation or volition are termed reflex actions. The mechanism in- 

 volved in every reflex action consists of at least the following struc- 

 tures (Fig. 207) : 



1. A sentient surface; e. g., skin, mucous membrane, sense 

 organ, etc. 



2. An afferent fiber and cell. 



