292 A Manual of Veterinary Physiology. 



stantly in a state of moderate activity, by which the 

 general arterial tone of the body is preserved: this forms 

 u contrast to the dilator fibres, which are only active 

 occasionally. 



If now we look at the vaso-dilator fibres we find that 

 they behave differently ; their origin from the cord is by 

 no means well known, but there is reason to believe that 

 they run in the roots of the motor-nerves to their destina- 

 tion, and do not pass through the sympathetic system as 

 we have found the constrictor fibres to do ; moreover, they 

 retain their medulla until near their termination. 



The constrictor fibres of the whole body are under the 

 control of a constrictor centre in the medulla known as 

 the vaso-motor, though under certain circumstances inde- 

 pendent centres may exist in the spinal cord. 



The dilator fibres are believed to have no special centre 

 in the central nervous system, but are affected by whatever 

 changes influence the centres of the motor-fibres they 

 accompany. 



If the vaso-constrictor nerves be divided, the vessels 

 become relaxed through the unbalanced action of the vaso- 

 dilators, and a considerable quantity of blood is sent to 

 the part. If the vaso-constrictor nerves be stimulated, the 

 vessels under its influence contract and press the blood 

 onwards, thus relieving the physiological congestion. No 

 better example of this can be found than in the nervous 

 supply of the submaxillary gland : the chorda tympani con- 

 tains fibres ^which act as vaso-dilators ; if the chorda be 

 stimulated, the vessels of the submaxillary gland dilate and 

 the veins pulsate ; if now the sympathetic be stimulated. 

 the vessels contract and the blood stream slows down tin 

 sympathetic therefore acts as a vaso-constrictor apparatus. 



Visceral Muscle Supply. — The next important function of 

 the sympathetic is the supply of nerve force to the involun- 

 tary muscular fibres of the body: in this way the peristaltic 

 movements of the stomach and bowels arc maintained. 

 These movements are independent of the brain and spinal 

 cord, and arc maintained by local ganglia. 



