THE CIRCULATION OF THE BLOOD AND LYMPH 159 



there are subsidiary centres all along the cord, and while a 

 very large number of the constrictor fibres are related to the 

 chief centre in the medulla, some are either normally under 

 the control of subordinate centres, or may in special circum- 

 stances come to be dominated by them. 



Thus, in the frog it is possible to go on destroying more 

 and more of the cord from above downwards, and still to 

 obtain reflex vaso-motor effects, as seen in the vessels of 

 the web, by stimulating the central end of the sciatic nerve. 

 Although these effects indeed diminish in amount as the 

 destruction of the cord proceeds, yet a distinct change can 

 be caused when only a small portion of the cord remains 

 intact. 



Similarly, in the mammal evidence has been obtained of 

 the existence of * centres ' at various levels of the cord, 

 capable of acting as vaso-motor centres after the chief 

 centre in the bulb has been cut off. For example, after 

 section of the cord at the upper limit of the lumbar region, 

 erection of the penis, which is known to be due to a reflex 

 dilatation of its arteries through the nervi erigentes, can still 

 be caused by mechanical stimulation of the glans penis, so 

 long as the afferent fibres of the reflex arc contained in the 

 nervus pudendus are intact. Destruction of the lumbar 

 cord abolishes the effect. It is impossible to avoid the con- 

 clusion that a vaso-dilator or erection centre, which is in 

 relation on the one hand with the nervi erigentes, and on 

 the other with the nervus pudendus, exists in the lower 

 portion of the spinal cord. Vaso-motor centres for the 

 hind-limbs have also been located in the same region. And 

 such centres appear to exist even beyond the limits of the 

 central nervous system. For when the lower portion of 

 the cord is completely destroyed, the dilatation of the 

 vessels of the hind-limbs, which is at first so conspicuous, 

 passes away after a time ; and the only plausible explanation 

 seems to be that the functions of vaso-motor centres have 

 been assumed by some of the peripheral (sympathetic) 

 ganglia (Goltz and Ewald). 



Of the anatomical relations of the nerve-cells that make up the 

 bulbar and spinal vaso-motor centres, little more is known than may 



