462 



SYMPATHETIC NERVE. 



stoppage of the heart's action when the cere- 

 bellum, or corpora quadrigemina were the 

 parts to which the stimulus was applied : 

 when applied to the medulla oblongata, on 

 the other hand, this effect was invariably pro- 

 duced. The cervical part of the spinal cord, 

 when stimulated, gave different results. In a 

 mouse, which had been under the influence 

 of the narcotic for 2i minutes, the heart was 

 repeatedly made to cease pulsating when the 

 wires were applied upon either side of the 

 spinal cord in the region of the third to the 

 fourth cervical vertebra, and also when ap- 

 plied to the part between the first and second 

 cervical vertebra. After repeating this ex- 

 periment several times, and with the same 

 result, he cut the spinal cord across in the 

 region of the second to the third vertebra ; 

 when the stimulus was now applied to the lower 

 cut extremity of the cord the heart's action 

 was accelerated. The cessation produced by 

 application of the stimulus to this part of the 

 spinal cord in the former experiment was, 

 therefore, according to Valentin, probably 

 due to its being transferred along the spinal 

 cord to the medulla oblongata. In two other 

 animals it was found that the two lower 

 thirds of the cervical portion of the cord in 

 like manner gave rise to no cessation in the 

 heart's action, but rather, after the first few 

 seconds, caused it to be accelerated. A 

 young rabbit was strangled, the head se- 

 parated from the body at the articulation 

 between the occipital and first cervical ver- 

 tebrae, and artificial respiration kept up. When 

 the wires of the battery, moderately loaded, 

 were now applied to the upper part of the 

 spinal cord, in the region of the first cervical 

 vertebra, the heart, which was before at 

 rest, commenced pulsating. The spinal cord 

 was laid bare from the first cervical to the 

 eighth thoracic vertebra. When the wires 

 were inserted in the region of the fifth cer- 

 vical to the second thoracic vertebra, the 

 heart's action was distinctly accelerated. 

 When the spinal cord was removed, the same 

 result still followed upon application of the 

 wires, because the roots of the nerves were 

 stimulated. When the heart was cut out of 

 the body, and again placed in situ, the above 

 experiment was repeated without effect. 



Just as stimulus of the sympathetic branches 

 in the mammalia is followed by acceleration 

 of the heart's action, while stimulus of the 

 pneumogastric causes it to cease pulsating, so 

 also Valentin concludes, from the above ex- 

 periments, that stimulus applied to the spinal 

 cord gives rise to the former result, while 

 from stimulus applied to the medulla oblongata 

 the latter result ensues. In the frog, accord- 

 ing to Valentin, the spinal cord has no in- 

 fluence over the movements of the heart. He 

 also holds, as already stated, that in this 

 animal, the sympathetic, in like manner, exer- 

 cises no influence in this respect. 



In a pigeon, he found that when the wires 

 of the magneto-electric apparatus were inserted 

 into the cerebellum, the heart's action became 

 more or less laborious : when applied to the 



spinal cord, in the region of the first cervical 

 vertebra, forwards, towards the medulla ob- 

 iongata, the heart's action was repeatedly 

 brought to a stand. 



The cessation in the heart's action by appli- 

 cation of the galvanic stimulus to the medulla 

 oblongata most readily ensues, according to 

 Valentin, when the wires are applied to its 

 sides, or to the under surface in the vicinity 

 of the roots of the eighth pair, and in no in- 

 stance does it ensue when the wires are ap- 

 plied to any part of the central nervous 

 masses after removal of the medulla oblongata. 



The influence exercised upon the heart's 

 action by the central nervous masses is also 

 shown by the diminution in the number as 

 well as in the strength of its pulsations, which 

 ensues when these are removed, especially on 

 removal of the medulla oblongata. That the 

 diminution in question does not depend en- 

 tirely upon the stoppage of the respiratory 

 process consequent on the destruction of the 

 medulla oblongata, has been shown by Budge. 

 When, in the frog, the anterior portion of the 

 medulla is left, the lungs continue to act ; and 

 yet, according to him, the pulsations of the 

 heart diminish very rapidly both in strength 

 and in frequency. He finds that, although 

 removal of the other parts of the central ner- 

 vous masses produces little immediate effect on 

 the heart's action, it seldom continues for any 

 length of time after the removal of the medulla 

 oblongata. The effects which follow disease 

 of these parts in like manner illustrate the in- 

 fluence which they exercise over the move- 

 ments of the heart. In compression of the 

 brain, as well as from lesion of the upper part 

 of the spinal cord, the pulsations are frequently 

 diminished : the effects of shock in altogether 

 stopping its action also illustrate the same 

 thing. 



From the experiments above mentioned, 

 Valentin and others hold that the nervous 

 centre upon which the heart's action depends 

 is the medulla oblongata. The particular 

 rhythmical order in which its different parts 

 contract is due, according to some, to pecu- 

 liarities in the manner in which they are acted 

 upon by the blood, the contact of arterial with 

 the lining membrane of the left cavities of the 

 organ, that of venous blood with the lining 

 membrane of those of the opposite side, fur- 

 nishing the proper stimuli, in obedience to 

 which these parts contract. The successive 

 contraction of auricles and ventricles is in like 

 manner explained by the blood first entering 

 the former, and causing them to contract. By 

 their contraction it is propelled into the ven- 

 tricles, and stimulates these to contraction 

 also, while the contraction of the ventricles 

 causes the auricles to become again filled with 

 blood from the veins, and so on indefinitely. 

 This rhythmical order in the movements of 

 the organ has also been attributed to pecu- 

 liarities in the mode of arrangement of its 

 muscular fibres. The muscular fibres of which 

 it is composed, as may be seen on examin- 

 ing with the microscope the auricles in the 

 heart of the frog or other small animal, do not 



