276 EXPEKLMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



lower end of the divided cord be then irritated, tlie vaso-motor 

 nerves which pass through it from the medulla to the body are 

 stimulated, and the blood-pressure rises. 



V. Vaso-iiihihitoi^y Nerves. Irritation of these nerves is con- 

 ducted to the vaso-motor centres, and acts on tliem in such a 

 way as to cause a reflex dilatation of the small vessels, either 

 (1) throughout the whole body, or (2) in one particular part 

 of it. 



1. Tlie chief nerve which causes dilatation throuohout the 

 whole body is one which runs from the heai't to the mediilla, 

 and is called, from its power of diminishing blood-pressui-e, 

 the depressor nerve. Its fibres seem to be included in the 

 vagus in the dog; but in the rabbit it generally runs separate 

 from the heart to the level of the thyroid cartilage; here it 

 divides into two so-called roots, one root going to the superior 

 laryngeal, and the other to the vagus nerve. These are generally 

 called roots, though, as the nerve conveys impressions from the 

 heart to the brain, they are, physiologically, really branches. 

 There seem to be also depressor fibres in the vagus itself; 

 but this nerve contains fibres of many kinds, and, among 

 others, some which cause contraction of the vessels and rise 

 of blood-pressure — hence called pressor- fibres. The former 

 seem to act on the vaso-motor system through the medulla 

 itself, while the latter affect it through a centre in the brain, so 

 that, when the brain is perfect, irritation of the central end of 

 the vagus causes increased contraction of the vessels and raised 

 blood-pressure ; but, when the brain is removed or its functions 

 abolished by opium, it causes dilatation of vessels and 

 diminished pressure. 



2. When a sensory nerve is irritated, the action of the vaso- 

 motor centre is suspended in the part supplied by the nerve, 

 and in those which immediately adjoin it, so that their vessels 

 become dilated, while at the same time contraction of the vessels 

 m other parts of the body is produced. The blood-pressure is 

 thus increased generally, and produces in the locally dilated 

 vessels a very rapid stream of blood. This fact was first dis- 

 covered, and its importance in therapeutics indicated, by 

 Ludwicj and Loven. 



