794 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



contracted through the reflection of that impulse from the spinal cord 

 through the efferent vaso-motor nerves to the muscles composing the 

 walls of the arterioles. In vaso-motor reflex action phenomena of inhibi- 

 tion are likewise capable of demonstration. Thus, the vaso-motor centre 

 in the body, which is a purely reflex centre, may be inhibited by stimuli 

 passing through certain nerves. If the so-called depressor nerve be 

 stimulated the vaso-motor centre in the medulla is inhibited and dilata- 

 tion of the blood-vessels ensues, as is evidenced by the great fall of blood 

 pressure. So, also, certain nerves when stimulated lead to a dilatation 

 of the blood-vessels by inhibition, in all probability, of the ganglia con- 

 tained within their walls. Such a reflex inhibition of vaso-motor action 

 is seen in the case of the chorda tympani, already referred to, in the reflex 

 stimulation of the nervi errigentes, and a number of other instances. 



As an example of changes in nutrition due to reflex action, illustra- 

 tions are not as readily found ; what is spoken of as sympathy is. an 

 example, however, of changes in nutrition of reflex nature. Surgeons 

 are well aware that when disease of an inflammatory character exists in 

 one eye it is not unusual to find the eye of the other side becoming 

 affected in a similar manner, purely in a reflex manner ; as a proof of this 

 may be mentioned that extirpation of the diseased eye is almost invariably 

 followed b}' a cure of the disease in the remaining one. So, also, section of 

 the supraorbital branch of the fifth pair of nerves leads to disturbances in 

 the cornea which are of a nutritive character and which are probably 

 due to some disturbances of the reflex control of nutrition. 



It has been already mentioned that in the spinal cord are located a 

 number of collections of cells which have for their function the control 

 of certain complicated co-ordinate movements ; such centres exert their 

 action in a reflex manner by the modification which they produce on the 

 afferent impulses brought to them. These centres retain their activity 

 even after the spinal cord has been removed from the medulla by section, 

 but all are to a certain extent controlled by the action of higher reflex 

 centres found in the medulla oblongata and cerebrum. 



The following represents the most important of these collections of 

 nerve-centres : — 



First, vaso-motor centres which control the calibre of the blood- 

 vessels are found in the floor of the fourth ventricle of the medulla 

 oblongata and distributed throughout the entire spinal axis, as is 

 evidenced by the fact that the dilatation of the blood-vessels, which 

 follows division of the spinal cord below the medulla is only transitory, 

 the blood-vessels regaining their normal tone as the shock of the opera- 

 tion passes off. If, however, the spinal cord be entirely destroyed, the 

 blood-vessels become then permanently paralyzed. 



It is probable that in the cerebro-spinal axis are likewise . found 



