104 NERVOUS SYSTEM. 



How do the vaso-motor fibres arise? 



They probably arise from spinal centres, and are controlled in 

 some way by the vertebral ganglia of the sympathetic system. 

 The vaso-dilator fibres are not to be distinguished as such, though 

 their presence is inferred from the action of special nerves. For 

 example, the stimulation of the chorda tympani nerve has a vaso- 

 dilator eifect in the submaxillary gland. The inhibitory and aug- 

 mentary effects of the cardiac nerves are similarly carried by or 

 through sympathetic plexuses. 



Do the visceral sympathetic nerves have a similar central 



origin ? 



Yes, but they too are acted upon by the ganglia through which 

 the fibres pass, and in which new nerves arise from the nerve-cells. 



What control has the sympathetic system over the secreting 



glands ? 



There has been demonstrated in some of the secreting glands 

 and it is probably true for all that functional stimuli, distinct 

 from the vaso-motor, come through the sympathetic nerves, and 

 that these fibres are closely associated with the vaso-motor fibres. 

 Thus, in the stomach the secretion of the gastric juice is only tem- 

 porarily suspended by the section of the vagi, and is resumed by 

 the action of the sympathetic, showing that the control is by the 

 sympathetic. 



What is " arrest of action " ? 



It is a temporary check upon the control of an organ by the 

 sympathetic system, which check comes through the cerebro-spinal 

 system. Such an action is frequent in the control of the glandular 

 organs, and usually is shown in the dilatation of the capillaries 

 through the arrest of the action of the vaso-motor nerves in response 

 to a sensory reflex. It is also seen in the sphincters, which keep 

 up a condition of tonic contraction to close the orifices of the body, 

 but relax in response to an impulse from the proper centre. The 

 sphincter ani in this way retains its hold upon the intestinal orifice 

 until the centre in the cord permits an arrest of action and relaxa- 

 tion of the muscle. 



SPINAL CORD. 

 Describe, roughly, the spinal cord. 



The spinal cord is that portion of the cerebro-spinal nervous sys- 



