ELEMENTARY EXPERIMENTAL PHYSIOLOGY 69 



nerve-fibres from the vagus have their cell-stations in these ganglia. The 

 sympathetic cardiac fibres have their cell-stations in the 3rd sympa- 

 thetic 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 tonus of the blood-vessels, 

 and possibly the frequency of the heart and the re'spiration. The 

 centrifugal cardiac nerves influence ^-~ ^f^S*^ - - Vs - H y- Br - 



the frequency and force of the 

 cardiac contraction and the con- 

 ductivity of the cardiac muscle 

 (chrono-, ino-, and dromo-tropic of 

 Engelmann). The inhibitory fibres 

 run in the vagus and arise from a 

 centre in the spinal bulb which is 

 in tonic action and curbs the heart. 

 The function of the vagus is to and Phillips.) 

 decrease the frequency, force, 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 heart-beat, 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. 70) 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. 



