740 NERVOUS SYSTEM. 



ments." These statements express pretty much all that we know of the origin of the 

 vaso-motor elements and their decussation, as far as their direct action is concerned ; but 

 some important points have been developed by observations upon reflex vaso-motor phe- 

 nomena, involving a transmission of impressions to the centres through the nerves of 

 general sensibility. 



Reflex Phenomena operating through the Sympathetic System. We shall not discuss, 

 in this connection, the reflex phenomena of secretion, as these have already been consid- 

 ered with sufficient minuteness, nor again treat of reflex action, through the sympathetic, 

 upon the general circulatory system, which has been taken up fully under the head of the 

 depressor-nerve of the circulation, but we shall here describe certain reflex acts, involv- 

 ing vaso-motor phenomena, which we thus far have touched upon very briefly. 



As regards animal heat, the phenomena of which are intimately connected with the 

 supply of blood to the parts, we may mention the observations of Brown-Sequard and 

 Lombard, who found that pinching of the skin on one side was attended with a diminu- 

 tion in the temperature in the corresponding member of the opposite side, and that some- 

 times, when the irritation was applied to the upper extremities, changes were produced 

 in the temperature of the lower limbs. Tholozan and Brown-S6quard found, 'also, that 

 lowering the temperature of one hand produced a^considerable depression in the heat of 

 the other hand, without any notable diminution in the general heat of the body. Brown- 

 Sequard showed that, by immersing one foot in water at 41 Fahr., the temperature of the 

 other foot was diminished about 7 Fahr. in the course of eight minutes. These facts 

 show that certain impressions made upon the sensory nerves affect the animal heat by 

 reflex action. As section of the sympathetic filaments increases the heat in particular 

 parts, with an increase in the supply of blood, and their galvanization reduces the quan- 

 tity of blood and diminishes the temperature, it is reasonable to infer that the reflex action 

 takes place through the vaso-motor nerves. If we assume that the impression is conveyed 

 to the centres by the nerves of general sensibility, and that the vessels are modified in 

 their caliber and the heat is affected through the sympathetic fibres, we have only to 

 determine the situation of the centres which receive the impression and generate the 

 stimulus. These centres, as we have already seen, are not situated in the sympathetic 

 ganglia, but in the cerebro-spinal axis. 



The existence of vaso-motor nerves and their connection with centres in the cerebro- 

 spinal axis are now sufficiently well established. It is certain, also, that centres presiding 

 over particular functions may be distinctly located, as the genito-spinal centre, in the spinal 

 cord opposite the fourth lumbar vertebra, and the cilio-spinal centre, in the cervical region 

 of the cord. A stimulus generated in these centres, sometimes as the result of impressions 

 received through the nerves of general sensibility, produces contraction of the non-striated 

 muscular fibres of the iris, vasa deferentia, etc., including the muscular walls of the blood- 

 vessels. The contraction of the muscular walls of the vessels is tonic ; and, when their 

 nerves are divided, relaxation takes place, and the vessels are dilated by the pressure of 

 blood. By this action, the local circulations are regulated in accordance with impressions 

 made upon sensory nerves, the physiological requirements of certain parts, mental emotions, 

 etc. Secretion, the peristaltic movements of the alimentary canal, the movements of the 

 iris, etc., are influenced in tjiis way. This action is also illustrated in cases of reflex pa- 

 ralysis, in inflammations as the result of "taking cold," and in many pathological condi- 

 tions, of which it is not our province to treat. The facts already noted with regard to 

 the excito-motor action of the spinal cord in the functions of animal life have their anal- 

 ogy in the vaso-motor reflex system. When the centres are destroyed, when the sensory 

 nerves are paralyzed by anaesthetics, or when the true vaso-motor nerves are divided, 

 reflex vaso-motor action is abolished. 



The vaso-motor filaments are not confined to the branches of the sympathetic, but 

 they exist as well in the ordinary cerebro-spinal nerves. Bernard has demonstrated this 



