314 TEXT-BOOK OF PHYSIOLOGY 



the heart-muscle is independent of its connection with the central organs of 

 the nerve system, and that it will continue to contract in a rhythmic manner 

 for a variable length of time even after its removal from the body of the 

 animal, the length of time varying with the animal and the conditions to 

 which it is subjected; that the stimulus is myogenic and chemic in character, 

 the result of a reaction between the chemic constituents, organic and in- 

 organic, of the muscle-cells and those in the lymph by which they are sur- 

 rounded. It has also been further shown that even in the living animal 

 the heart will continue to beat and fulfil its functions after division of all 

 nerves in connect'on with it. A dog thus experimented on lived for eleven 

 months, and beyond the fact of becoming fatigued more readily upon exer- 

 tion than formerly, exhibited no striking disturbance of its functions. 



Intra-cardiac Nerve-cells. In all classes of vertebrate animals, 

 groups of nerve-cells are present in certain portions of the heart which 

 bear an anatomic and physiologic relation to the heart-cells on the one 

 hand, and to the nerves connecting them with the central organs of the 

 nerve system on the other hand. 



In the frog heart a group of nerve-cells is found in the sinus at its 

 junction with Jthe auricle, known as the crescent or ganglion of Remak; 

 a second group is found at the base of the ventricle on its anterior aspect, 

 and known as the ganglion of Bidder; a third group is found in the 

 auricular septum, known as the septal ganglion, or the ganglion of v. 

 Bezold or of Ludwig. The majority of the cells are situated on the surface 

 of the heart just beneath the pericardium. From the cell-body fine non- 

 medulla ted fibers pass into the substance of the heart, to become his- 

 tologically and physiologically related with the muscle-fiber. 



In the dog heart and* in the mammalian heart generally, though nerve- 

 cells are present, they are not arranged in such definite groups, but are more 

 widely , distributed in the terminations of the venae cavae, pulmonic veins, 

 the walls of the auricles, and in the neighborhood of the base of the ventricles. 



Extra-cardiac Nerves. The extra-cardiac nerves which connect the 

 heart with the central nerve system and through which the activities of the 

 heart are influenced are two: viz., the sympathetic and the vagus or pneumo- 

 gastric. Experimental investigation has established the fact that the sympa- 

 thetic is the accelerator nerve to the heart, the nerve which accelerates the 

 rate and augments the force of the normal beat; while the vagus is the 

 inhibitor nerve, the nerve which inhibits or controls the rate and the force 

 of the beat. For this reason these two nerves will be considered in the 

 order stated. The course of the fibers composing these nerves, from their 

 origin to their termination, and the relation they bear to one another and 

 to neighboring structures, vary somewhat in different animals. 



The Origin and Distribution of the Sympathetic Nerves in Mammals. 

 As the heart-muscle belongs to the group of autonomic tissues, it follows 

 that the accelerator nerve-fibers consist of two consecutively arranged 

 and histologically related neurons, the first of which originating in the 

 central nerve system is termed pre-ganglionic, the second of which originat- 

 ing in a sympathetic ganglion is termed post-ganglionic ; this latter 

 neuron constitutes the sympathetic nerve proper. 



The pre-ganglionic fibers have their origin in the medulla oblongata and 

 very probably from nerve-cells in the gray matter beneath the floor of the 



