THE CIRCULATION OF THE BLOOD 313 



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 connection 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. 

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

 in all classes of vertebrate animals, 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. 



Intra-cardiac Nerve-cells. In the frog heart a group of nerve-cells 

 is found in the sinus at its junction with the 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 gan- 

 glion 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-medullated fibers pass into the substance of the heart, to 

 become histologically 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, pulmonary 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 motor 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 

 in accordance with the necessities of blood distribution. For this reason 

 these two nerves will be considered in the order stated. The course < 

 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. 

 -The sympathetic nerve-fibers which influence the action of the heart a 

 connected on the one hand with the heart-muscle itself and on the o 

 hand with nerve-fibers coming from the central nerve system. 

 are non-medullated and post-ganglionic, the latter medullated an 



fibers have their origin in ^f 



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

 fourth ventricle. From this origin they descend the spinal cord as far as the 

 level of the second, third, and at times the fourth thoracic nerves. At 1 



