The study of the action of drugs on invertebrata has not been carried out methodically to any great extent, but it offers a very promising field for investigation, and probably in the course of a few years may yield very valuable results.

Action of Drugs upon Medusae

This subject has been worked at, almost exclusively by Romanes 1 and Krukenberg.2 At present it has little practical bearing, but it promises to be of great service by enabling us to understand better the action of drugs on contractile structures generally, and on the heart in particular.

In medusae the swimming organ consists of a bell-shaped mass of contractile substance, within which the polyp hangs like the clapper. Around the margin of this bell are a number of ganglia connected with one another by nervous filaments, and forming a peripheral ring.

Bell..................

Lithocyst and ganglion Tentacles ............

Fig. 29.   Medusa (Sarsia), natural size.

Polypite.

Fig. 29. - Medusa (Sarsia), natural size.

In the normal state of the animal, the bell alternately contracts and dilates rhythmically, so that the animal is propelled through the water.

When the marginal strip containing the ganglia is removed, the bell becomes entirely motionless. The bell thus resembles, as we shall see afterwards, the ventricle of the frog's heart, both in the relation of ganglia to it, and in its rhythmical movements. Oxygen accelerates, and carbonic acid slows and finally stops, the rhythmical movements.

When the bell, paralysed by the removal of the ganglia which supply its normal stimulus to motion, is momentarily stimulated by a single induction shock, it invariably responds by a single contraction.

1 Romanes, Phil. Trans. vol. clxvi. part 1, and vol. clxvii.part 2,1866 and 1867.

2 Krukenberg, Vergleichend. physiologische Studien, Heidelberg, 1880.

When successive shocks are employed at regular intervals the effect of each increases until the maximum is reached (Fig. 30, cf. pp. 122 and 123).

Fig. 30.   Shows the increasing contractions of the tissue of the medusa when stimulated by repeated weak induction shocks of the same intensity.

Fig. 30. - Shows the increasing contractions of the tissue of the medusa when stimulated by repeated weak induction shocks of the same intensity. The first two shocks had no apparent effect, and the first feeble contraction seen in the figure was caused by the third shock. (From a paper by Romanes in Phil. Trans.)

But if an additional constant stimulus is supplied to it by the addition of acid to the water in which it is floating; by the passage of a constant or of an interrupted electrical current through it; or by alcohol or glycerine dropped upon its surface, it commences to beat regularly, rhythmically, and continuously. When rhythmical action is thus artificially induced in the paralysed bell, its rate is increased by raising the temperature, and reduced by cooling it. Temperatures below 20° or above 85° arrest the rhythm.

When the marginal strip containing the ganglia is cut off and left attached only at one point, a stimulus applied to its end travels along the strip and finally causes the bell to contract. The stimuli which pass along may be

Strip of contractile tissue with fringe of tentacles..........

tissue cut from the bell, but left attached at one end.

Fig. 31.   Diagram of a medusa (tiaropsis), about one third natural size, with a strip of contractile of two kinds   they may occur separately or together.

Fig. 31. - Diagram of a medusa (tiaropsis), about one-third natural size, with a strip of contractile of two kinds - they may occur separately or together. The first kind is a wave of contraction in the contractile tissue of the strip itself. If the stimulus is applied to a portion of the strip the contraction will pass along like a wave until it reaches the bell, which it excites to contraction. The second is a rudimentary form of nervous activity. This may occur along with the contraction wave, and when this is the case it is seen to pass along in front of the contractile wave. But it may also occur when no wave of contraction takes place. Its occurrence is rendered visible by the movements of the tentacles which fringe the strip and are much more sensitive than the contractile tissue of the strip itself. This wave of stimulation without contraction passing along the strip will cause the bell to contract on reaching it, provided there is a marginal ganglion in the bell, but not if the bell is paralysed. The wave of stimulation is more easily excited than that of contraction, so that it may occur from stimuli too weak to excite a wave of contraction. The passage of stimuli along the strip may be impeded or prevented altogether by compressing the strip, by making transverse incisions into it so as to narrow the band of tissue by which the wave is transmitted, or by injuring the tissue by straining. Sometimes the contraction wave may be blocked by the injury before the stimulus wave, and sometimes the stimulus wave may be blocked before the contraction wave. When the block is only partial it frequently happens that two or three waves will pass along the strip up to the block without being able to cross it, but after a long time their effect appears to penetrate so that a wave at last crosses it.

As Gaskell has shown, a similar occurrence takes place in the frog's heart, and stimuli proceeding from the auricle to the ventricle may also be blocked by compression.

The influence of poisons can be studied either upon the bell containing the ganglia, or upon this marginal strip.