103 PKACTICAL PHYSIOLOGY 



the cardiac rhythm. Extra-cardiac nerve-fibres from the vagus have 

 their cell-stations in these ganglia. The sympathetic cardiac fibres 

 have their cell-stations in the 3rd sympathetic ganglion in the frog, in 

 the stellate or 1st thoracic ganglion in the mammal. Non-medullated 

 nerve-fibres spin a fine network through all parts of the cardiac muscle. 

 A great many of the cardiac nerve-fibres are centripetal or afferent, and 

 convey impulses up the vagi to the spinal bulb, which reflexly control 

 the touus of the blood-vessels, and possibly the frequency of the heart 

 G. c. Vs. Hy. Dr. and the respiration. The centri- 

 fugal cardiac nerves influence the 

 frequency and force of the cardiac 

 contraction and the conductivity and 

 excitability of the cardiac muscle. 

 The inhibitory fibres run in the 

 vagus and arise from a centre in the 

 spinal bulb which is in tonic action 



Fio. 106.-l)iagram of nerves in the frog's an d curbs the heart. The function 

 neck. Dissection from behind. (Pembrey 



aud Phillips.) of the vagus is to decrease the fre- 



gjigncy, fo^rce, conductivity and excitability. The sympathetic fibres, 

 which arise in the mammal from the anterior spinal roots in the upper 

 thoracic region, antagonise the action of the vagus. The vagus, by 

 reducing the heajtJ^ea^ causes anabolism, and the sympathetic kata- 

 bolism of the cardiac muscle. The after-effect of vagus excitation is 

 increased energy of contraction, while that of the sympathetic is exactly 

 the opposite. The function of the cardiac nerves is to co-ordinate the 

 beat of the heart to the needs of the body, and to co-ordinate the 

 functions of the other organs of the body to the needs of the heart. 



Dissection of Vago-Sympathetic Nerve from behind. The skin in 

 the mid-line of the back is divided and the scapula lifted up and cut 

 away. The fore-limb is pulled outwards and then removed. A small 

 plug of paper is placed in the frog's mouth to put the parts on the 

 stretch. In front of the divided brachial plexus (Br., Fig. 106) there 

 can be seen (Hy.) a much smaller nerve the hypoglossal which is the 

 first spinal nerve in the frog and passes down to the floor of the mouth ; 

 (VS.) the vago-sympathetic, which can be traced from the skull, and 

 runs by the side of the carotid artery (C.) and crosses underneath the 

 hypoglossal nerve ; (G-.) the glosso-pharyngeal. This nerve issues with 

 the vago-sympathetic nerve, but soon turns downwards and forwards 

 to the floor of the mouth. The glossopharyngeal and hypoglossal 

 nerves are then cut and a small piece of the bone containing the 

 foramen from which the vago-sympathetic nerve issues is cut away from 

 the skull. By means of this piece of bone the vago sympathetic nerve 

 can at any time be lifted up without damage and laid upon electrodes. 



