CHAP, iv.] THE VASCULAR MECHANISM. 213 



(in satisfactory experiments) undiminished intensity, even when the 

 whole of the brain, down to a certain limit in the medulla oblongata, 

 has been removed. But if the removal be carried beyond this 

 limit, or if a small area of the medulla oblongata lying above the 

 calamus scriptorius be removed, the effect on the general blood- 

 pressure of stimulating the central stump of the sciatic we might 

 add, of any other afferent nerve is comparatively insignificant. The 

 simplest view to take of these facts is to suppose that this small 

 portion of the medulla oblongata acts as a vaso-motor centre, by the 

 action of which ordinary afferent impulses coming from the sciatic 

 or any other afferent nerve, are transformed into vaso-motor 

 impulses of constructive (or as in the case of an animal under 

 chloral, of dilating) effect and so discharged along the splanchnic 

 nerves. 



The lower limit of this region which we may call the medullary 

 vaso-motor centre has been placed 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 4rnm. higher up, i.e. 

 about 1 or 2 mm. below the corpora quadrigemina. When trans- 

 verse 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 effect of 

 stimulating the sciatic becomes 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 of grey 

 matter, called by Clarke the antero-lateral nucleus, and containing 

 large multipolar cells. 



Whether this medullary vaso-motor centre has any distinct 

 automatic action, whether it may be regarded as continually gene- 

 rating out of its own molecular oscillations, and discharging along 

 the vaso-motor fibres, impulses whereby the general arterial tone 

 is maintained, is a question which, like the allied question mooted 

 on p. 188, need not be discussed here. Granting even the existence 

 of such automatic functions, they must be of secondary importance. 

 As we have already urged, the great use of the whole vaso-motor 

 system is not to maintain a general arterial tone, but to modify 

 according to the needs of the economy the condition of this or 

 that vascular area. 



The impulses passing down the vaso-motor fibres of the cervical 

 sympathetic and of many other nerves may similarly be traced back 

 to this same region of the medulla oblongata. Whether all vaso- 



