NERVES OF THE HEART. CARDIAC MUSCLE. 263 



not completely empty themselves at each stroke; they some- 

 times do so more completely and sometimes less. In this 

 manner the auricles can to a great extent control the work 

 done by the ventricles, through influencing the amount of 

 blood in the latter at the commencement of the ventricular 

 systole : more complete relaxation of the auricles during i 

 diastole promotes inflow from the great veins, more extensive 

 contraction during auricular systole more completely fills the 

 ventricles. As we shall see, the force and rate of the auricular 

 beat is much more under the control of nerves reaching the 

 heart from other parts than is that of the ventricles. The 

 auricles are a feed-pump adjusting their work, and through 

 it the work of the whole heart, to the general condition of 

 the Body; the ventricles are a grosser force-pump driving on 

 whatever blood is supplied to them, be it much or be it little. 

 The Extrinsic Nerves of the Mammalian Heart. As in 

 the frog, these come from two sources, at least so far as indi- 

 cated by gross anatomy. Their exact anatomical arrangement 

 differs in various mammals, as the rabbit, dog, and man, and 

 even somewhat in different individuals of these species, but in 

 the main is the same. The pneumogastric gives off from its 

 main stem in the neck several cardiac branches; so do the 

 lower cervical and the upper thoracic ganglia of the sympa- 

 thetic chain. Both sets intermingle, and near the heart end 

 in plexuses containing nerve-cells; from these plexuses nerves 

 are distributed to that organ. In the heart itself, as already 

 stated, are collections of ganglion-cells in the auricles near the 

 ends of the great veins, near the base of the ventricles, and a 

 few cells scattered over the ventricles even in their apical re- 

 gions. The nerve-fibres coming through the pneumogastrics 

 are medullated and consist of a set of small fibres and a group 

 of large: the smaller lose their medulla in ganglion-cells in or 

 near the heart; the larger retain the medullary sheath, and may 

 be traced even over the ventricles, which in this respect differ 

 from that of the frog; the fibres supplied from the sympathetic 

 are non-medullated. Broadly speaking, the nerve-fibres fall 

 into three physiological sets corresponding to the three 

 anatomical varieties: the small medullated fibres are effer- 

 ent and inhibitory when excited they slow the heart-beat; 

 the large medullated are in part at least afferent, conveying 

 to the central nervous system impulses which originate in the 

 heart; the sympathetic fibres are efferent and excitor, and 



