An American text-book of physiology / by Henry P. Bowditch [and others]., edited by William H. Howell.

  • William Henry Howell
Date:
1897
Catalogue details

Licence: Public Domain Mark

Credit: An American text-book of physiology / by Henry P. Bowditch [and others]., edited by William H. Howell. Source: Wellcome Collection.

Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.

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    before we fan judge of the irritability and the efU'ct of various influences upon it, we must consider how far the activity of the nerve and muscle is depend- ent on the cliarai'ter, strength, and method of application of the irritant. Conditions which Determine .the EflQciency of Irritants.—Some of these conditions can be best studied on nerves, while others are more ap- parent in their effects on muscles. The most useful irritant for ])urposes of study is the electric current. INIechanical, thermal, and chemical irritants are likely to injure the tissue, and are not manageable, whereas electricity, if not too strong, can be applied again and again without producing any permanent alteration, and can be accurately graded as to strength, place, time, and dura- tion of application, etc. Of course the results obtained by the use of a given irritant cannot be accepted for others until verified. The conditions which determine the eifectiveness of the electric current as an irritant may be classed as follows: (a) The rate at which the intensity changes. (6) The strength of current. (c) The density of current. {d) The duration of application. (e) The angle of application. (/) The direction of flow. Irritating Effect of the Electric Current.—Galvani, in seeking to find the effect of atmospheric electricity upon the animal body, suspended frogs by copper wires from an iron balcony, and observed the remarkable fact, that when the wind blew the legs against the balcony the muscles of the frogs twitched. He repeated the experiment in his laboratory, and concluded that the frogs had been excited to action by electric currents developed within them- selves ; he looked upon the metals which he had used merely as conductors for this current. Volta, Professor of Natural Philosophy at Pavia, repeated Gal- vani's experiment, and concluded that there had been an electric current developed from the contact of the dissimilar metals with the moist tissues of the frog. In accordance with this idea he constructed the voltaic pile, and this was the starting-point of the electric science of to-day. Although it is true that, under certain conditions, differences in electric potential sufficient to excite muscles to contraction can be developed in the animal body, the contractions of the frog's leg which Galvani observed were due to the metals which he employed. The experiment can be easily per- formed by connecting a bit of zinc to a piece of curved copper wire, and bring- ing the two ends of the arc against the moist nerve and muscle of a frog. A stronger and more efficient shock can be obtained from a Daniell or some other voltaic cell. A Daniell cell (Fig. 5) is composed of a zinc and copper plate, the former dipping into dilute sulphuric acid, the latter into a strong copper-sulphate solution. Although gravity will keep these liquids separated, if the cell is to be moved about it is better to enclose one of them in a porous cup. A common form of cell consists of a glass jar, in the middle of which is a porous cup; outside the cup is the sulphuric acid and the
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