THE BRAIN. 487 



possesses automatic centers which generate and emit impulses 

 independently of stimuli from without. 



Respiratory Center. This center is situated in the floor of the 

 fourth ventricle, and when injured, respiration ceases immediately. 

 Some authorities place it among the reflex centers. It may, in- 

 deed, be excited reflexly, but there are reasons for believing it to 

 possess automatic powers as well. If the spinal cord is divided 

 below the medulla, although the respiratory movements of the 

 thorax cease, those of the nose and larynx continue. Under these 

 circumstances no afferent impulses can be transmitted through 

 the spinal nerves, and the only channel is the cranial nerves ; 

 but if, while the medulla and cord are left undisturbed, the 

 cranial nerves are cut, respiration continues. These two series 

 of experiments show that respiration will continue independently 

 of stimuli from without that is, automatically. 



The principal nerves that transmit the efferent impulses pro- 

 ducing the respiratory movements are the intercostals to the inter- 

 costal muscles, and the phrenics to the diaphragm. The respiratory 

 center is double, so that one side may act after the other is injured. 

 Division of one phrenic paralyzes only the side of the diaphragm 

 to which it is distributed. The respiratory center may also be ex- 

 cited reflexly. The afferent fibers under these circumstances are 

 those of the pneumogastric. 



Cardio-inkibitory Center. In the cardio-inhibitory center are 

 generated those impulses which, travelling to the heart by the 

 vagus nerve, inhibit or restrain the action of that organ. These 

 fibers convey impulses to the heart and to the muscular fibers of 

 the superior vena cava, some of which diminish the frequency of 

 the heart's action, while others lessen the strength of its contrac- 

 tions. This center can be stimulated directly, as when the blood 

 is very venous or when the blood-pressure in the cerebral arteries is 

 increased. It may also be stimulated reflexly, as when the sensory 

 nerves of the abdominal viscera are irritated, as is shown in Goltz's 

 percussion experiment, by percussion of the abdomen of a frog. 



Cardio-accelerator Center. Fibers from this center, whose ex- 

 istence is not absolutely demonstrated, convey impulses to the 

 heart which increase the frequency of its beats, and there are also 

 fibers which transmit impulses that increase the force of the 

 systole. These fibers pass down the spinal cord and thence into 

 the sympathetic through the communicating branches of the in- 

 ferior cervical and the six upper thoracic nerves. 



Vasomotor Center. The principal vasomotor center is situated 

 in the medulla, in the floor of the fourth ventricle, extending from 

 its upper part to a point about 4 mm. from the calamus scriptorius. 

 When the center is destroyed there is a marked fall in arterial 

 blood-pressure, due to the loss of tone in the small blood-vessels. 

 After a while the pressure is increased under the influence of 

 stimuli sent out from the subsidiary vasomotor centers in the cord. 



