VASO-MOTOR CENTRES AND NERVES. 643 



All the vaso-motor nerves are derived from the medulla oblongata and 

 the spinal cord. Some of the vaso-motor fibres to the head pass in the 

 trunks of the motor cranial nerves, but most of them come from the ante- 

 rior roots of some of the spinal nerves and pass to the head by the filaments 

 of distribution of the cervical sympathetic. The vaso-motor fibres pass in 

 the lateral columns of the cord, and from the cord, in the anterior roots of 

 the spinal nerves, in the dog, as far down as the second pair of lumbar 

 nerves. These fibres are medullated but are of small size. They pass to 

 the blood-vessels either through branches from the sympathetic ganglia or 

 through the ordinary cerebro-spinal nerves. They are therefore not confined 

 to branches of the sympathetic, as Bernard has shown by the following ex- 

 periment : He divided the fourth, fifth, sixth, seventh and eighth pairs of 

 lumbar nerves on one side in a dog, at the spinal column, and paralyzed mo- 

 tion and sensation in the leg of that side, but the temperature of the two 

 sides remained the same. He afterward exposed and divided the sciatic 

 nerve on that side, and then noted decided increase in temperature. This 

 experiment, which is only one of a large number, shows that the ordinary 

 mixed nerves contain vaso-motor fibres, which are entirely independent of 

 the nerves of motion and sensation, a fact which is now well known to physi- 

 ologists and has frequently been illustrated in cases of disease in the human 

 subject. 



The vaso-motor nerves are capable of influencing local circulations, 

 probably through distinct centres for different parts. Direct stimulation of 

 the principal vaso-motor 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 influenced 

 by vaso-motor nerves, but there is little known of the mechanism of this action. 



Reflex Vaso-Motor Phenomena. The most important physiological acts 

 connected with 'the vaso-motor nerves are reflex. It is evident from experi- 

 ments 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 phenomena of which are intimately con- 

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

 servations of Brown-Sequard and Lombard, who found that pinching of the 

 skin on one side was attended with a diminution in the temperature 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 de- 

 pression- 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. 



