THE CIRCULATION OF THE BLOOD AND LYMPH 167 



speak of it as the vaso-constrictor centre, since it is undoubtedly 

 connected with most or all of the vaso-constrictor paths, and 

 has not been shown to be similarly connected with the vaso- 

 dilator paths. There is, indeed, not the same solid evidence for 

 the existence of a general vaso-dilator centre in the bulb as for 

 the existence of the general vaso-constrictor centre. Yet there 

 are facts which indicate that the bulbar vaso-motor centre or 

 centres, when reflexly stimulated, can, and often do, respond not 

 merely by an increase or a remission of vaso-constrictor tone, 

 but by a simultaneous inhibition of vaso-constrictor fibres and 

 excitation of vaso-dilators leading to a fall of pressure, or by a 

 simultaneous inhibition of vaso-dilators and excitation of vaso- 

 constrictors leading to a rise of pressure. 



The spinal cells of origin of the pre-ganglionic segments of the 

 vaso-constrictor paths constitute subordinate centres which 

 either normally support a certain degree of vascular tone, or 

 come to do so after the chief vaso-motor centre has been cut off. 

 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 eventually, if not at once, as vaso-constrictor centres after 

 the loss of the controlling influence of the bulb. The best example 

 of a vaso-dilator centre is that situated in the lumbar cord, which 

 controls the erection of the penis. After total section of the cord 

 at the upper limit of the lumbar region, erection, which is known 

 to be due to a reflex dilatation of the arteries of the organ through 

 the nervi erigentes, can still be caused (in dogs) by mechanical 

 stimulation of the glans penis, so long as the afferent fibres of the 

 reflex arc contained in the nervus pudendus are intact. Destruc- 

 tion of the lumbar cord abolishes the effect. It is impossible to 

 avoid the conclusion 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. When the brain, 

 the bulb, and the upper portion of the cord have been eliminated 

 by ligation of all the arteries from which blood can possibly 

 reach them, a sufficient vascular pressure persists to permit the 

 circulation to go on in the lower portion of the body for hours. 

 And while section or freezing (Fig. 68) of the cord in the lower 



