2. Sourer of contractility: whence it rire.1 This important question, like the last, is de bated up to the present da'y, brit seems at length to have become disenthralled of certain loose hypotheses which have long interfered with its settlement. The discussion may be limited to such particulars as seem to he the most conclusive.
It may be observed that the contractility and development of muscle, other things being the same, are always proportionate to one another. Allcauses interfering witlidevelopmentdiminish contractility. Thus muscles become atrophied and weak by disuse, by lessening their supply of brood, by cutting off their connexion with the central part of the nervous system. They are, on the contrary, augmented both in size and power by active use, during which both the vascular and nervous parts supplied to them are no doubt urged to increased activity. flow is it to be decided whether these changes of contractility depend on changes of nutrition, or whether both be not a common result of changes in the amount of nervous power brought to act upon the muscles. Dr. Marshall Dail has remarked that in paralysis from disease involving the spinal cord or nerves, the wasting of the muscles is far more rapid and complete than in paralysis from affection of the brain, wherein the spinal cord and its connection with the muscles remains in a normal state; and the deduction seems at first sight plain and inevit able, that it is from the spinal cord that the contractility is derived, or at least that eke integrity of the spinal system is essential to the maintenance of that property in the muscles. An ingenious experiment of Dr. John Reid's,* however, proves that this is not the case, and explains the part which the spinal system plays in respect of this property in the instances referred to. " The spinal nerves were cut across, as they lie in the lower part of the spinal canal, in four frogs, and both posterior extremities were thus insulated from their ner vous connexions with the spinal cord. The muscles of one of the paralyzed limbs were daily exercised by a weak galvanic battery, while the muscles of the other limb were allowed to remain quiescent. This was con tinued for two months, and at the end of that time the muscles of the exercised limb retained their original size and firmness and contracted vigorously, while those of the quiescent limb had shrunk to at least one-half of their former bulk, and presented a marked contrast with those of the exercised limb. The muscles of
the quiescent limb still retained their contrac tility, even at the end of two months ; but there can be little doubt (adds Dr. Reid) that from the imperfect nutrition of the muscles and the progressing changes in their physical structure, this would in no long time have dis appeared had circumstances permitted me to prolong the experiment." It is clear from this description that though contractility remained, it was diminished proportionally to the wasting, in the limbs that had not been exercised.
The results of this admirably devised experi ment cannot possibly be reconciled with the opinion that the spinal cord has any necessary or immediate influence in conferring contracti lity on muscles—that it is the source whence that property is derived. On the contrary, they show in a manner that admits of no dispute that both contractility and nutrition have been preserved together by the continued activity of the property existing in the fully developed organ at the period when the experiment was begun ; and hence it is plain, and conformable to all analogy, that contractility is a property depending for its integrity on a healthy state of nutrition, which in its turn requires for its support the due exercise of the property it confers.
It might, perhaps, be argued by one dis 'posed to uphold the electrical" hypothesis of the nervous influence and muscular power, that in the foregoing experiments the galvanism supplied the place of the intercepted nervous communication, by directly furnishing the muscles with the endowment of contractility ; and it is not easy to meet the objection by any decided proof to the contrary. It would be very difficult to induce oft-repeated contractions in a pasty: ed muscle by any other than elec trical agency ; but the refutation of this view will be found in the general arguments against the identity of the nervous influence with any form of electricity.