86 A MANUAL OF VETERINARY PHYSIOLOGY 



vascular areas which maintain and regulate the blood-pressure. 

 On entering the superior root of the spinal nerves, the fibres 

 travelling up the cord by means of collateral branches, make 

 connection with the vasomotor centre in the medulla. We have 

 now seen the channels by which this almost microscopic collec- 

 tion of cells in the medulla is brought into connection with the 

 vessels of the whole body. 



The impulses carried by afferent fibres to the vasomotor centre 

 are of two antagonistic kinds ; they either stimulate it and produce 

 contraction of the bloodvessels, with a consequent increase in 

 blood-pressure, or they diminish the tone of the centre, produce 

 a relaxation of the bloodvessels, and a fall in pressure. The 

 first fibres are known as pressor, the latter as depressor, terms 

 which correspond to the effect they produce on the blood-pressure. 



As previously stated, it is in the skin and abdominal viscera 

 that the impulses which regulate the maintenance of normal 

 arterial pressure originate ; there are supplemental sources, but 

 the above are the most important. The fibres passing from the 

 skin area are stimulated by the external temperature, a low 

 temperature causing impulses to be transmitted which contract 

 the vessels, a high temperature causing a relaxation of the vessels, 

 accompanied by congestion of the skin. But the splanchnic area 

 is even more actively important than the skin in regulating 

 blood-pressure, especially in those animals such as the herbivora, 

 where the alimentary canal is largely developed. If the 

 splanchnics be cut, the intestines become congested, in consequence 

 of dilatation of the bloodvessels, and there is a severe fall in 

 blood-pressure. If the peripheral end of the divided nerve be 

 stimulated, the vessels contract, and the blood-pressure at once 

 rises. 



The action of the constrictor fibres of the bloodvessels is 

 always more in evidence than that of the dilators, and the 

 reason of this can be readily understood by remembering that 

 the arterioles have always to be kept in a condition of contraction 

 if blood-pressure is to be maintained. 



Bernard's classical experiment of dividing the cervical sympa- 

 thetic in the rabbit produces not only a remarkable picture of 

 vasomotor effects, but illustrates the constrictor impulses which 

 are constantly passing to the bloodvessels. On division of the 

 sympathetic, the ear on that side suddenly becomes flushed with 

 blood, hot, and congested, and vessels not previously visible to 

 the naked eye now become very apparent ; and if the upper end 

 of the nerve be stimulated, so as to imitate roughly the impulses 

 passing along it in an intact condition, the vessels at once con- 

 tract, the flushed appearance disappears, and the ear becomes 

 cooler. 



