198 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



spinal nerve. According to Langley, 1 they do not differ from the pilo-motor 

 and secretory nerves except in the nature of the structure in which they termi- 

 nate. They are not interrupted by other nerve-cells on their course. The 

 action of the sympathetic vaso-motor cells is influenced by the vaso-motor 

 cells of the spinal cord and bulb. These are probably small cells situated at 

 various levels in the anterior horn and lateral gray substance. Their axis- 

 cylinder processes leave the cerebro-spinal axis by the anterior roots 2 of 

 certain spinal and by certain cranial nerves, and enter sympathetic ganglia, 

 where they end in terminal twigs probably in contact with the sympathetic 

 vaso-motor cells. The vaso-motor cells lying at various levels in the cerebro- 

 spinal axis are in turn largely controlled by an association of cells situated in 

 the bulb and termed the vaso-motor centre. The neuraxons (axis-cylinder 

 processes) of the cells composing this "centre" pass in part to the nuclei of 

 certain cranial nerves and in part down the lateral columns of the cord, to 

 end in contact with the spinal vaso-motor cells. The vaso-motor apparatus 

 consists, then, of three classes of nerve-cells. 3 The cell-bodies of the first class 

 lie in sympathetic ganglia, their neuraxons passiug directly to the smooth mus- 

 cle- in the walls of the vessels; the second are situated at different levels in 

 the cerebro-spinal axis, their neuraxons passing- thence to the sympathetic gan- 

 glia by way of the spinal and cranial nerves; and the third are placed in the 

 bulb and control the second through intraspinal and intracranial paths. The 

 nerve-cell of the first class lies wholly without the cerebro-spinal axis, the third 

 wholly within it, while the second is partly within and partly without, and 

 binds together the remaining two. 



The evidence for the existence of these vaso-motor nerve-cells must now 

 be considered. We shall begin with those of the third class, constituting the 

 so-called bulbar vaso-motor centre. 



Bulbar Vaso-motor Centre. — The section of the spinal cord near its 

 junction with the bulb is followed by the general dilatation of the blood- 

 vessels of the trunk and limbs. The dilated vessels are again constricted 

 when the severed fibres in the spinal cord are artificially stimulated. Hence 

 the section caused the dilatation by interrupting the vaso-constrictor impulses 

 passing from the bulb to parts below. The position of the bulbar vaso- 

 constrictor centre has been determined by Owsjannikow and Dittmar. The 

 former observer divided the bulb transversely at various levels. When the 

 section fell immediately caudal to the corpora quadrigeraina, only a slight 

 temporary rise in blood-pressure was observed. When, however, the section 

 fell a millimeter or two nearer the cord, a considerable and permanent fall in 

 the blood-pressure was noted. Further lowering was seen as the sections 

 were carried still farther toward the spinal cord, until at length, about four 

 millimeters from the corpora quadrigemina, no further fall took place. The 



' Langley ' f Physiology, 1894, xvii. p. 314. 



'Compare Werziloff: Centralblalt fur Physiologi' , ISO*), x. p. 194. 

 I'.\ "nerve-cells" is meant the cell-body with nil its processes, namely, the neuraxon, or 

 axis-cylinder process, and the dendrites, <>r protoplasmic processes. 



