Feb. 7, 1878] 



NATURE 



285 



but, on the contrary, that the possession of a closed 

 tracheal system is a secondary condition, derived from 

 ancestors provided with spiracles. 



He adopts the view that the existing insects are derived 

 from an ancestor, in which the larvae resembled the 

 existing genus Campodea, with a hemimetabalous meta- 

 morphosis, and an open tracheal system ; and he dwells 

 on the important fact that in Campodea each spiracle has 

 an independent set of tracheae. So also in the course of 

 embryonal development, the tracheal systems rise sepa- 

 rately, and then the anterior and posterior branches unite 

 to form the lateral ducts. 



In a still earlier stage he thinks it probable that the 

 tracheae resembled those of the curious genus Peripatus. 

 He observes that the skin-glands of certain worms secrete 

 not only fluid, but also gas (carbonic acid), and from this 

 to an absorbing function would be a comparatively small 

 step. He supposes, then, that the tracheae are derived 

 from the skin- glands of worms, passing firstly through the 

 stage now represented by Peripatus, in which there are a 

 number of tracheal tubes with numerous scattered open- 

 ings ; secondly, though one represented now by Campodea 

 and certain myriapods, in which the spiracles are 

 situated in pairs, and are connected with separate 

 tracheal systems. 1. L. 



ON THE EVOLUTION OF HEAT DURING 

 MUSCULAR ACTION "^ 



"DROF. A. FICK, of Wiirzburg, in continuing his 

 -*- researches on the source of muscular power, has 

 obtained some new and exceedingly important results, of 

 which the following is a condensed account : — 



It is obviously an interesting question in the phy- 

 siology of muscle what fraction of the work yielded by 

 chemical action in muscular tissue can be employed in 

 overcoming mechanical resistance? the remainder of 

 the chemical work appearing, in all probability, as heat. 



Many years ago Helmholtz calculated, from certain 

 considerations, into which, however, there entered several 

 hypothetical factors, that possibly one- fifth of the total work 

 yielded by chemical foice in the human body might be 

 employed in muscular action, the remaining four-fifths ap- 

 pearing as sensible heat. From this it necessarily follows 

 that a much larger proportion than one-fifth of the work 

 yielded by chemical force in the muscle itself can be 

 employed m overcoming mechanical resistance, inasmuch 

 as it is assumed that a great part of the oxidation takes 

 place in other tissues, where mechanical work is quite out 

 of the question, and where heat alone can be the result. 



If, however, thermodynamical experiments show that 

 of the chemical work going on in the muscle only a small 

 fraction, not much exceeding one-fifth, produces mechanical 

 effect ; then, supposing the coefficient of Helmholtz to be 

 true, it would be proved that only minute quantities of 

 combustible material are oxidised elsewhere than in the 

 muscles. The author's experiments have been made with 

 a view to answer the first of the above questions — what 

 fraction of the chemical force eliminated m the muscle is 

 used in mechanical work ? Such experiments can, of 

 course, with the present means of research, only be 

 carried cut upon the muscles of the frog. How far the 

 results obtained are applicable to other classes of animals, 

 is a distinct question. 



Thus two magnitudes have to be determined in absolute 

 measure, viz., the mechanical work performed by the 

 muscle, and secondly, the amount of chemical work that 

 the muscle has yielded during the action. 



The amount of heat produced in the muscle was of 

 course measured by multiplying the rise in temperature 

 of the muscle by its capacity for heat. In the calcula- 

 tions the specific heat of muscle was taken as equal to 

 that of water. It cannot be greater, and is probably not 



' Ueber die Warmeentwickelung bei der Muskelzuckun^," in the Ar- 

 ckiv.f. Fhysiologie, Band xvL 



much less, inasmuch as three-fourths of living muscle are 

 water. The rise in temperature was measured by thermo- 

 electrical means. The galvanometer used had no fixed 

 magnet, and its constancy was proved to extend over 

 many weeks, and even months. The thermopile had to 

 be so arranged that it was as much as possible surrounded 

 by the mass of muscle ; its construction will be better 

 understood after the preparation has been described. The 

 gastrocnemius muscle, which is the favourite preparation 

 in such experiments, was replaced by the masses of 

 muscle which pass from the pelvis to the tibia on the 

 inner side of each thigh, whilst the other muscles, with 

 the sartorius and biceps, as well as both the thigh-bones, 

 were removed. Then, on suspending the pelvis, the two 

 prepared masses of muscle hung vertically downwards in 

 intimate contact with each other, all the nerves belonging 

 thereto being easily preserved. One end of the thermo- 

 pile, with very flat and thin elements, was then placed in 

 the fissure between the two masses of muscle, this 

 arrangement being found by experience to be a perfectly 

 trustworthy one. 



A remark is necessary concerning the method of 

 irritating the preparation. Some years ago the author 

 had the opportunity of observing, in some unpublished 

 experiments, that an electric current of sufficient strength 

 to produce the most powerful contraction in a muscle, 

 does not appreciably raise the temperature of the latter. 

 Even with Heidenhain's exceedingly delicate thermopile 

 there was scarcely any evidence of heat being produced 

 in a dead muscle through which a current of twenty-four 

 Daniell's elements was passing for several seconds ; and 

 even induction currents of immense strength produced no 

 visible thermal effect. This fact is of great interest in 

 myothermic experiments, as it is thus no longer necessary 

 to impart the stimulus through the nerve, but simply to 

 subject the muscle to direct electrical irritation. 



In his experiments, the author has adopted preferen- 

 tially the method of direct irritation, one of the two copper 

 wires connected with the induction-coil being attached to 

 the pelvis, and the other to the knee of the frog. 



The mechanical work was measured by connecting the 

 preparation with one arm of a lever to which a weight 

 was attached, and, in some of the experiments, there were 

 also two balanced weights placed upon the lever to 

 increase its inertia, by which it was found that the work 

 performed was very considerably increased. 



The following is a summary of the chief results arrived 

 at by these experiments : — 



1. By the interposition of a thin thermopile between 

 suitable masses of muscle, it is possible to determine with 

 great accuracy the absolute amount of heat produced by 

 their contraction. 



2. The determination of the muscle-temperature is not 

 interfered with by electrical currents, which, for the par- 

 pose of irritation, are passed through the muscle. There- 

 fore direct electrical irritation of the muscle is permissible, 

 and indeed far preferable, in myothermic researches. 



3. To the fundamental law of Heidenhain, that a muscle 

 contracting to its greatest extent evolves more heat the 

 greater its initial tension, we may now add that, with 

 equal initial tension, a muscle will evolve more heat if, by 

 means of weights in equilibrium, greater tension be pro- 

 duced during the contraction. 



4. A muscle overcoming a greater resistance, works not 

 only with more activity but also with more economy than 

 when occupied in a smaller effort. 



5. In an energetic muscular contraction against as 

 great a resistance as possible the eliminated chemical 

 iorce is about four times as great as the mechanical work 

 it performs. With a less resistance the chemical is a 

 greater multiple of the mechanical force, and with no 

 resistance at all it is obviously indefinitely greater. 



6. The amount of heat produced by the eliminated 

 chemical force in an energetic contraction of i grm. of 



Q2 



