THE FROG, AND ON THE ACTION OF THE VAGUS NERVE. 
1027 
diminution of the force of the contractions of the cardiac muscle, and acceleration of 
that rate to augmentation of that force, we can say that the action of the vagus upon 
all parts of the heart is similar ; and that the difference of its action at different times 
depends not upon a multiplicity of nerve fibres, each possessing different functions, but 
rather upon the different effects produced by the same nerve in consequence of varia¬ 
tions in the condition of nutrition of the heart as a whole—variations of nutrition 
to which naturally the more delicately-organised ganglion cells would prove more 
sensitive than the comparatively coarser muscular fibres. 
How, then, can we imagine to ourselves the action of the nerve upon the heart ? In 
the first place, it is clear that the nerve cannot be considered as the motor nerve of 
the muscle in the sense in which the words motor nerve are ordinarily used. The 
nerve itself cannot upon stimulation cause a contraction of the cardiac muscle ; but 
causes such a modification in the chemical processes going on in the muscle as to 
modify the force of the contractions which take place when the impulses from the 
motor ganglia reach the muscle. In fact, two distinct processes are going on—the one, 
motor, represented by the impulses from the motor ganglia, and corresponding to the 
blow by which such an explosive substance as nitro-glycerine, for instance, is exploded ; 
the other, trophic, represented by the action of the vagus upon the muscular sub¬ 
stance, and corresponding to something affecting the nature or properties of the 
explosive substance itself. 
The problem, then, to be explained is, what conception can be formed of the nature 
of the changes caused in the muscle by the action of the nerve, which shall satisfy the 
conditions that the same nerve can produce in different cases a graduated series of 
different effects extending from a simple increase in the force of the contractions on 
the one side down to diminution of force even to complete standstill on the other ? 
In trying to form a theory to explain new facts, it is always advisable to see if any 
theory which has been devised to explain somewhat similar facts will apply to this case 
also. With this consideration I cannot help thinking that the recent views which 
have been put forward to explain the changes going on in gland cells in a condition of 
rest and activity may help in the conception of the changes going on in the cardiac 
muscle cells under the influence of such a nerve as the vagus. In all secretory glands it 
appears to be the fact that the formation of the ultimate products of secretion is a 
gradual process, three stages of which can be recognised, viz.: the growth of the proto¬ 
plasm, the formation of zymogen from the protoplasm, and the formation of the ultimate 
products from the zymogen ; and according to Langley* these three processes not only 
go on at the same time, but also there may be, under different circumstances, a dif¬ 
ference in the relative rates at which they proceed. 
If, then, we imagine in the cardiac muscle an initial or ground substance, or, in corre¬ 
spondence with the gland cell, a muscle protoplasm and a final or explosive substance 
which is not self-explosible, but requires a stimulus to fire it off. then we may conceive 
Phil. Trans., Part III., 1881. 
