CARDIAC AND VASO-MOTOR NERVES. 275 



depressor, or (e) of the vagus of the other side. Eeflex irritation 

 is only likely to be caused by drugs having a powerful local 

 action. 



Drugs may also paralyse the inhibitory fibres, and thus 

 quicken the heart. 



III. Quiclcenmg Nerves. These belong to the sympathetic 

 system. They have their origin in the brain or medulla, pass 

 down through the cervical part of the spinal cord to the last 

 cervical and first dorsal ganglion (which are often united), and 

 thence through the third branch of the ganglion to the heart. 

 Quickening fibres are said by some to run also in the cervical 

 part of the sympathetic cord. Unlike the vagus, the quick- 

 ening nerves are not normally in constant action. They may 

 be irritated — 



1. By the direct action of drugs upon them. 



2. Indirectly by the drugs producing a diminished blood- 

 pressure, which acts as a stimulus to them. 



ly. Vaso-motor Nerves, which cause the smaller arteries, and 

 probably also the capillaries, to contract. These belong to the 

 sympathetic system ; and the most important of them are the 

 splanchnics, which produce contraction of the intestinal vessels. 

 As these vessels can, under certain circumstances, hold all the 

 blood in the body, the influence of the splanchnics over the 

 blood pressure is very great ; and division of these can lower 

 it, or stimulation of them increase it very much. The centre 

 for the whole vaso-motor system, however, seems to be in 

 the medulla oblongata ; and it is generally in constant action, 

 keeping up a certain amount of contraction or tone in these 

 vessels. Its activity may be increased, and the vessels made 

 to contract — 



1. By direct irritation of the centre. 



2. By reflex irritation through («) the cervical sympathetic 

 (h) the vagus, when the brain is intact, and the animal not 

 narcotised, (c) sensory nerves. When the medulla is separated 

 from the rest of the body by dividing the spinal cord at the 

 atlas, it can, of course, no longer exert any influence over the 

 vessels ; and they consequently become dilated throughout the 

 whole body, and the blood pressure sinks very low. If the 



T 2 



