CHAP, iv.] THE VASCULAR MECHANISM. 341 



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 reflex 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. 



But as we have seen the instances in which we have clear and 

 direct evidence of vaso-dilator fibres, as distinguished from those 

 in which the evidence is indirect and sometimes not decided, are 

 on the whole few. In some of these cases the flushing of the 

 organs by means of vaso-dilator fibres is a very special act, 

 securing a very special purpose. This is notably the case with 

 the nervi erigentes ; and in the dog which uses its mouth and 

 especially the tongue as a means of cooling the body, we may 

 recognise an advantage in the tongue and other parts of the 

 mouth being provided with distinct vaso-dilator fibres. But the 

 object of the special supply to the salivary glands is not so clear ; 

 for these glands are singular in this respect, since we have not in 

 the case of other glands or of the glandular walls of the alimentary 

 canal similarly sharp evidence of distinct vaso-dilator mechanisms. 



173. 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 vasormotor 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 



