THE CIRCULATION OF THE BLOOD. 303 



is coincidently an impairment in the irritability and contractility of the heart- 

 muscle. Extra systoles, however, may have their origin in the auricular 

 walls as well. 



If a series of successive stimuli be thrown into the heart-muscle the 

 effect will vary in accordance with their time intervals. Should this be less 

 than about three seconds there will be a gradual increase in the height for 

 some half dozen contractions, a result to which the term "staircase" or 

 "treppe" has been given. This increase in the height of the contraction 

 is attributed to an increase in the irritability and contractility of the muscle 

 the result of the primary stimulating action of fatigue products. 



THE NERVE MECHANISM OF THE HEART. 



By this term is meant a combination of nerves and nerve-centers which 

 cooperate to increase or decrease either the rate or force or both of the 

 heart's contraction in accordance with the needs of the system. That the 

 heart is normally influenced by the central organs of the nerve system in 

 response to the action of nerve impulses reflected to them from many organs 

 of the body is a matter of personal experience; that it is abnormally influenced 

 by the same or other organs in response to nerve impulses reflected to them 

 in consequence of pathologic and traumatic processes occurring in different 

 regions of the body, and that both heart and nerves are modified in different 

 ways by the action of drugs introduced into the body, are matters of daily 

 clinical experience. 



The nerves comprising this mechanism and the relation they bear one to 

 another are represented in Fig. 140. 



It was stated in a previous paragraph, page 289, that the contraction of 

 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 in origin and chemic in 

 character, the result of a reaction between the chemic constituents, organic 

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

 surrounded. 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 v. Bezold's 

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



