THE SPINAL CORD. 



505 



their efficiency as quickly responsive organs is increased. That such a 

 slight contraction is present even in a state of rest is shown by the fact that 

 if a muscle be divided in the living animal the two portions will contract 

 and separate to a certain distance. The condition of the muscle was 

 formerly attributed to an automatic and continuous discharge of energy from 

 the nerve-cells. 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 stimulus 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, (see page 54.) 



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

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



,sp.c 



FIG. 233. DIAGRAM SHOWING THE STRUCTURES INVOLVED IN THE PRODUCTION or REFLEX 

 ACTIONS, G. Bachman. r.s. Receptive surface; af.n. afferent nerve; e.c. emissive or motor cells in 

 the anterior horn of the gray matter of the spinal cord, sp.c.; ef.n. efferent nerves distributed to 

 responsive organs, e. #., directly to skeletal muscles, sk.m., and indirectly through the interme- 

 diation of sympathetic ganglia, sym.g., to blood-vessels, b.v., and to glands, g. The nerves 

 distributed to viscera are not represented. 



probably reflex in origin, dependent on the arrival of afferent impulses from 

 the periphery. It is probable that future investigation will disclose the 

 existence and pathway of these afferent fibers. 



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

 spinal cord are capable of receiving and transforming afferent nerve impulses, 

 the result of peripheral stimulation, into efferent nerve impulses, which are 

 transmitted outward to skeletal muscles, exciting contraction; to glands, 

 provoking secretion; to blood-vessels, changing their caliber; and to organs, 

 inhibiting or augmenting 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 involved in every reflex action 

 consists of at least the following structures (Fig. 233) : 



