THE PONS VAROLII 543 



8. The cardie-inhibitory centers. The medulla contains the centers which 

 maintain the proper rhythm of the heart, these centers acting through the vagus 

 fibers. These terminate in a local intrinsic mechanism which has been al- 

 ready discussed. It is claimed that the center can be stimulated directly, as 

 by the condition of the blood circulating within it. It is constantly 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 re- 

 flex center. Sensory or afferent impulses arriving over the sensory paths in 

 the vagus itself, by abdominal paths through the sympathetic, 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 ol 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. 



n. Centers for the secretion of sweat exist in the medulla. The medullary 

 centers control the subsidiary spinal sweat centers. They may be excited un- 

 equally 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 medul- 

 lary reflex centers would be able to coordinate 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 constantly in- 

 fluence the details of the reactions. The activities are unconscious reflexes 

 in the same sense that the motor reflexes of the spinal cord are unconscious 

 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 with the two halves of 

 the cerebellum and which connect the bulb and spinal cord with the upper 

 part of the brain. It must not be forgotten that the pons contains several 

 smaller collections of nerve cells. Sections reveal the following parts or struct- 

 ures, beginning from 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- 



