Upon the foregoing facts, and others already mentioned - viz., that the impulse starts in the nerve from different poles and with different force, with a making and a breaking shock - depends the law of contraction, which would be difficult to understand without bearing in mind all these interesting points.

It was found that, with the same strength of stimulation, not only were different degrees of contraction produced with making and breaking shocks, but also that, other things being similar, a different result followed when the current was sent through the nerve in an upward direction (i. e., from the muscle), and when it was sent in a downward direction (i. e., toward the muscle). The stimulating current is spoken of, in the former case, as an ascending current, and in the latter as a descending current. The following is a tabular view of the law of contraction: -

Ascending Currents.

Descending Currents.

Weak Stimulation.

Make = Contraction. Break = No Response.

Make = Contraction. Break = No Response.

Medium "

Make = Contraction. Break = Contraction.

Make = Contraction. Break = Contraction.

Strong "

Make = No Response. Break = Contraction.

Make = Contraction. Break = No Response.

To explain this law, the following points must be kept in view: i. In a breaking shock, it is the disappearance of anelectrotonus which causes the stimulation to start from the anode.

2. In a making shock, it is the appearance of catelectrotonus which causes the stimulation to start from the cathode.

3. With the same current the make is more powerful than the break.

4. Anelectrotonus causes reduction of irritability and conductivity of the nerve.

5. Catelectrotonus causes increase of irritability and conductivity of the nerve.

6. With ascending currents the part of the nerve next the muscle is in a state of reduced functional activity (anelectrotonus).

7. With descending currents the part of the nerve next the muscle is in a state of exalted activity (catelectrotonus).

8. The reduction or exaltation of activity is much greater with strong currents. That only making shocks cause contraction with very weak currents, simply depends on the greater efficacy of the entrance of catelectrotonus into the nerve, which causes the makingstimulation.

That contraction follows in all four cases, with medium stimulation, is explained by assuming that the depression of the functional activity of the nerve is not sufficient to affect its conductivity.

The want of response to a making shock, in the case of the strong descending current, depends upon the fact that the part of the nerve near the muscle, around the anode, is in a state of lowered activity, and is, therefore, unable to conduct the impulse which has to pass through this region from the cathode, where the stimulation takes place, in order to reach the muscle.

The absence of contraction at the breaking of a strong descending current, is caused by the same lowering of the conductivity of the nerve between the point of stimulation and the muscle, because at the cessation of strong catelectrotonus, the region near the cathode rebounds from exalted to depressed activity, and at the moment of stimulation the greater part of the intra-polar region is anelectrotonic.

The special function of nerve fibres may be briefly stated to be their power' of rapidly intercommunicating between distant parts. The axis cylinder has undergone a special development, by which it is enabled to conduct impulses much more quickly than ordinary protoplasm. Each muscle tissue transmits impulses about thirty times more slowly than a nerve fibre. A highly-organized animal body, without nerve fibres, would be in a worse condition than a highly-organized state without a telegraphic or even a postal system.