REFLEX VASOMOTOR PHENOMENA 615 



probably through distinct centres for different parts. Direct stimula- 

 tion of the principal vasomotor centre (10 to 12 or more single induction 

 shocks per second for strong currents or 20 to 25 for moderate currents) 

 increases the blood-pressure to the maximum. 



The contractile coats of the veins and lymphatics probably are influ- 

 enced by vasomotor nerves, but little is known of the mechanism of this 

 action. 



Reflex Vasomotor Phenomena. The most important physiological 

 processes connected with the vasomotor nerves are reflex. It is evident 

 from experiments on the inferior animals and observations on the human 

 subject that there are afferent as well as efferent nerves. The reflex 

 acts connected with secretion have already been considered ; but there 

 are other phenomena that demand a brief description. 



As regards animal heat, the production of which is intimately con- 

 nected with the supply of blood to the parts, it is important to note the 

 observations of Brown-Sequard and of Lombard, who found that pinch- 

 ing of the skin on one side was attended with a diminution in the tem- 

 perature in the corresponding member of the opposite side, and that 

 sometimes, when the irritation was applied to the upper extremities, 

 changes were produced in the temperature of the lower limbs. Tholozan 

 and Brown-Sequard 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. (5 C.) the temperature of the other foot was diminished by about 

 7 Fahr. (4 C.) in the course of eight minutes. These experiments 

 show that certain impressions made on 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 faradization reduces the quantity of blood and diminishes 

 the temperature, it is reasonable to infer that the reflex action takes 

 place through the vasomotor nerves. If it is assumed that the impres- 

 sion is conveyed -to the centres by the nerves of general sensibility, 

 and that the vessels are modified in their calibre and the heat is affected 

 through the sympathetic fibres, it remains only to determine the situa- 

 tion of the centres that receive the impression and generate the stimulus. 

 These centres are situated in the cerebro-spinal axis. 



The existence of vasomotor 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 acts may be dis- 

 tinctly located, as the genito-spinal centre, in the spinal cord opposite 

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



