278 USE OF VASO-DILATOR FIBRES. [Book r. 



the chorda tympani and other nerves to the salivary glunds and, 

 by dilating the blood vessels, seems a copious tiow of blood 

 .through the glands while, as we shall see later on, they excite 

 the glands to secrete. The centre of this reflex action appears 

 to lie in the spinal bulb and may be thrown into activity not 

 only by impulses reaching it along the specific nerves of taste, 

 but also by impulses passing along other channels ; thus, emotions 

 started in the brain by the sight of food or otherwise may give 

 rise to impulses passing down along the central nervous system 

 itself to the spinal bulb, or events in the stomach may send 

 impulses up the vagus nerve, or stimulation of one kind or another 

 may send impulses up almost any sentient nerve, and these 

 various impulses reaching the spinal bulb may, by reflex action, 

 throw into activity the vaso-dilator fibres of the chorda tympani 

 and other analogous nerves, and bring about a flushing of the 

 salivary glands, while at the same time they cause the glands to 

 secrete. 



The vaso-dilator fibres of the nervi erigentes may be thrown 

 into activity in a similar re tie x way, the centre, which is also 

 easily thrown into activity by impulses descending down the spinal 

 cord from the brain, being placed in the sacral and perhaps also 

 in the upper lumbar or lower thoracic region of the spinal cord. 

 That such a centre does exist is shewn by the fact that, when 

 in a dog the spinal cord is completely divided in the thoracic 

 region, erection of the penis may readily be brought about by 

 stimulation of appropriate sentient surfaces. And other instances 

 might be quoted in which vaso-dilator fibres appear as part of a 

 reflex mechanism the centre of which is placed in the central 

 nervous system not far from the origin of the nerves in which the 

 vaso-dilator fibres run. 



§ 151. Turning now to the vaso-constrictor fibres we find 

 that these form a more coherent system ; and this is in accordance 

 with the feature of the vaso-constrictor mechanisms, that they are 

 largely employed to produce general effects Moreover their utility 

 is increased, though at the same time their use becomes somewhat 

 more complicated, by reason of the existence of tonic influences ; 

 since the same fibres may, on the one hand, by an increase in the 

 impulses passing along them, be the means of constriction, and 

 on the other hand, by the removal or diminution of the tonic 

 influences passing along them, be the means of dilation. We have 

 already traced all the vaso-constrictor fibres from the middle 

 region of the spinal cord to the sympathetic system in the thorax 

 and abdomen ; from thence they pass (1) by the splanchnic, 

 hypogastric, and other nerves to the viscera of the abdomen and 

 pelvis, (concerning the vaso-motor nerves of the thoracic viscera 

 we know at present very little), (2) by the cervical sympathetic 

 to the skin of the head and neck, the salivary glands and mouth, 

 the eyes and other parts, and possibly the brain including its 



