5/6 THE NERVOUS SYSTEM 



been already discussed. It is claimed that the center can be stimulated 

 directly, as by the condition of the blood circulating within it. It is con- 

 stantly exerting a tonic influence over the heart, which is the chief reason for 

 considering it an automatic center. But the cardio-inhibitory center is 

 primarily a reflex center. Sensory or afferent impulses arriving over the 

 sensory paths in the vagus itself, by abdominal paths through the sympa- 

 thetic, and through cutaneous nerves, are constantly causing reflex discharges 

 of inhibitory impulses from this center. 



9. Accelerator centers for the heart are present in the medulla. They 

 are reflexly stimulated by sensory impulses arising from the same general 

 source as in the preceding center. 



10. Vaso-motor centers which control the unstriped muscle of the arteries, 

 are also situated in the medulla. The nerve cells constituting the center 

 are under the constant influence of nerve impulses flowing in from the sensory 

 and motor structures throughout the whole body. The reflexes produced by 

 the afferent impulses bring about the variations in vaso-motor tone that not 

 only regulate the general vascular responses of the body, but control and co- 

 ordinate the local changes in the size of the blood vessels. 



11. Centers for the secretion of sweat exist in the medulla. The medul- 

 lary centers control the subsidiary spinal sweat centers. They may be 

 excited unequally so as to produce unilateral sweating. 



The reflex medullary centers described above are comparable to the spinal 

 reflex centers previously described. If the medulla were completely isolated 

 from the higher cerebral centers, and the spinal cord removed with the ex- 

 ception of those paths which are necessary to maintain respiration, these 

 medullary reflex centers would be able to co-ordinate afferent impulses in the 

 same general way that isolated segments of the cord do. In the living body, 

 however, the medullary centers are under the influence of changes going on 

 in regions of the nervous system both above and below, changes which con- 

 stantly influence the details of the reactions. The activities are unconscious 

 reflexes in the same sense that the motor reflexes of the spinal cord are un- 

 conscious and machine-like. The main difference is one of complexity and 

 not of kind. 



The Pons Varolii. The pons Varolii is generally spoken of as a great 

 commissure of fibers; of fibers which connect the two halves of the cerebellum 

 and which connect the bulb and spinal cord with the cerebellum. It must not 

 be forgotten that the pons contains several smaller collections of nerve cells. 

 Sections reveal the following parts or structures, beginning with the anterior 

 or ventral surface. 



i. Transverse or commissural fibers connect one side of the cerebellum 

 with the other through the middle peduncle. These fibers connect the cere- 

 bellar cortex with the cells of the pontine nuclei; some are afferent, some 

 efferent; some end in the gray matter of the pons on the same side near the 



