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TEXT-BOOK OF PHYSIOLOGY. 



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directly to the heart-muscle, but to the nerve-cells, around the body 

 of which they end in basket-like formations. The fibers in the vagus 

 are pre-ganglionic; those of the nerve-cells post-ganglionic. (See / 

 Fig. 128.) 



The Sympathetic. V-Histologic investigation has shown that the 

 sympathetic nerves which pass to the heart are non-medullated. 

 Experiment has shown that they are also efferent in' functionJv}vThe 

 fibers are peripherally coursing axons of nerve-cells situated in the 

 ganglion stellatum. After reaching the heart they may terminate 

 directly in the muscle-cell or indirectly through the intervention of 

 the heart nerve-cells. The former method is the more probable. 

 The nerve-cells in the ganglion stellatum are in relation with small 

 medullated nerve-fibers which emerge from the cord in the anterior 

 roots of the second and third thoracic nerves, pass through the white 

 rami communicantes, and thence to the ganglion stellatum, where 



their end branches arborize around the 

 nerve-cells. The nucleus of origin of 

 these medullated fibers is probably in 

 the medulla oblongata. The fibers 

 emerging from the cord are pre- 

 ganglionic, those emerging from the 

 ganglion, post-ganglionic. 



In the frog these two sets of nerve- 

 fibers, instead of passing to the heart 

 along separate paths, as in man and 

 mammals, generally pass along the 

 same path and in the same sheath. 

 The sympathetic fibers proper, the 

 post-ganglionic fibers, arise from 

 nerve-cells in the third sympathetic 

 ganglion. From this origin they 



ascend, passing successively through the second sympathetic gan- 

 glion, the annulus of Vieussens, the first sympathetic ganglion, to 

 the ganglion on the trunk of the vagus, at which point they enter 

 the sheath of this nerve. For this reason the common trunk which 

 descends to the heart is generally spoken of as the vago-sympathetic 

 nerve. The pre-ganglionic fibers emerge from the cord in the ante- 

 rior roots of the third spinal nerve, pass through the rami communi- 

 cantes to the third sympathetic ganglion, around the cells of which 

 the nerve-fibers arborize. 



Physiologic Action of the Vagus. The information now pos- 

 sessed regarding the influence which the central nerve system 

 exerts upon the heart has been derived largely from experiments 

 made on the nerves of the frog, toad, and turtle. To demonstrate 



Vagus 



..Sympathetic Neuron 



Mvscle 

 Cell 



FIG. 128. DIAGRAM SHOWING 

 THE RELATION OF THE VAGUS 

 TO THE HEART MUSCLE-CELL. 



