226 THE VASCULAR MECHANISM. 



while slowly falls again, the fall sometimes beginning to appear before the 

 stimulus has been removed. There can be no doubt that the rise of pres- 

 sure is due to the constriction of certain arteries ; the arteries in question 

 being those of the abdominal splanchnic area certainly, and possibly those 

 of other vascular areas as well. The effect is not confined to the sciatic ; 

 stimulation of any nerve containing afferent fibres may produce the same 

 rise of pressure, and so constant is the result that the experiment has been 

 made use of as a method of determining the existence of afferent fibres in 

 any given nerve and even the paths of centripetal impulses through the 

 spinal cord. 



If, on the other hand, the animal be under the influence not, of urari,' 

 but of a large dose of chloral, instead of a rise of blood-pressure, a fall 

 quite similar to that caused by stimulating the depressor is observed when 

 an afferent nerve is stimulated. The condition of the central nervous 

 system seems to determine whether the effect of afferent impulses on the 

 central nervous system is one leading to an augmentation of vaso-constrictor 

 impulses and so to a rise, or one leading to a diminution of vaso-constrictor 

 impulses and so to a fall of blood-pressure. 



162. We have used the words " central nervous system " in speaking 

 of the above ; we have evidence, however, that the part of the central nervous 

 system acted on by the afferent impulses is the vasomotor centre in the 

 medulla oblongata, and that the effects in the way of diminution (depressor) 

 or of augmentation (pressor) are the results of afferent impulses inhibiting 

 or augmenting the tonic activity of this centre or of a part of this centre 

 especially connected with abdominal splanchnic nerves. The whole brain may 

 be removed right down to the medulla oblongata, and yet the effects of stimu- 

 lation in the direction either of diminution or of augmentation may still be 

 brought about. If the medulla oblongata be removed, these effects vanish 

 too, though all the rest of the nervous system be left intact. Nay, more, by 

 partially interfering with the medulla oblongata, we may partially diminish 

 these effects and thus mark out, so to speak, the limits of the centre in ques- 

 tion within the medulla itself. Thus, in an intact animal under urari, stimu- 

 lation of the sciatic nerve with a stimulus of a certain strength will produce 

 a rise of blood-pressure up to a certain extent. After removal of the whole 

 brain right down to the medulla oblongata, the same stimulation will produce 

 the same rise as before ; the vasomotor centre has not been interfered with. 

 Directly, however, in proceeding downward, the region of the centre in ques- 

 tion is reached, stimulation of the sciatic produces less and less rise, until at 

 last when the lower limit of the centre is arrived at no effect at all on blood- 

 pressure can be produced by even strong stimulation of the sciatic or other 

 afferent nerve. In this way the lower limit of the medullary vasomotor 

 centre has been determined in the rabbit at a horizontal line drawn about 

 4 or 5 mm. above the point of the calamus scriptorius, and the upper limit 

 at about 4 mm. higher up i. e., about 1 or 2 mm. below the corpora quadri- 

 gemina. When transverse sections of the brain are carried successively 

 lower and lower down, an effect on blood-pressure in the way of lowering it 

 and also of diminishing the rise of blood-pressure resulting from stimulation 

 of the sciatic, is first observed when the upper limit is reached. On carrying 

 the sections still lower, the effects of stimulating the sciatic become less and 

 less, until when the lower limit is reached no effects at all are observed. The 

 centre appears to be bilateral, the halves being placed not in the middle line, 

 but more sideways and rather nearer the anterior than the posterior surface. 

 It may perhaps be more closely defined as a small prismatic space in the 

 forward prolongation of the lateral columns after they have given off their 

 fibres to the decussating pyramids. This space is largely occupied by a mass 



