Page:Popular Science Monthly Volume 75.djvu/64

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.

believe that these effector elements are the most primitive members in the typical neuromuscular mechanism.

That there is absolutely no trace of nervous activity in sponges is probably not true, but their extreme inertness shows that this function is certainly in a most primitive state and corresponds at best probably only to that sluggish form of reception and transmission that Kraft (1890) demonstrated for ciliated epithelium and that is probably characteristic of other epithelia. Taking all in all, the only element of the neuromuscular mechanism that is really present in sponges is the effector as represented by the sphincters of the pores and oscula.

If independent effectors occur in sponges, it is not unlikely that they may be present in the higher animals, and as possible examples of these the sphincter pupillæ of the eye in vertebrates and the heart-muscle may be considered. The sphincter pupillæ is a ring of muscle imbedded in the iris and surrounding the pupil in the eyes of most vertebrates. Its contraction would naturally reduce the size of the pupil and thereby diminish the amount of light that enters the eye. In the higher vertebrates it is well known that this reaction has the character of a simple reflex in which the retina is the receptor, with the optic nerve as its transmitting organ, and the stem of the brain is the adjustor from which the oculomotor nerve transmits peripherally to the effector, the sphincter pupillæ. In the lower vertebrates, particularly in the fishes and amphibians, it has long been known that the sphincter pupillæ will react in a characteristic way even in extirpated eyes. This fact has been explained by those who cling to the idea of a reflex as due to intraocular nervous connections between the retina and the sphincter. But Steinach (1892) demonstrated the contraction of the pupil in the extirpated eyes of lower Vertebrates from which the retina had been removed and moreover he showed that when a minute beam of light was thrown on a part of the sphincter, that part contracted first and was followed later by the rest of the muscle, an observation recently confirmed by Hertel (1907) in the eyes of higher vertebrates, including man. It therefore seems quite certain that the sphincter pupillæ of the vertebrate eye, though usually controlled by nerves, is a muscle that can be directly stimulated and in this respect is an independent effector like the sphincters of the pores in sponges.

A second case of independent muscle action in the higher metazoans is the heart-muscle. This muscle for a long time past has been the occasion of much discussion. In the vertebrates it is still an open question whether the beat of the heart is primarily nervous or muscular in its origin and the neurogenic and the myogenic theories of heart action have had a lengthy history (Engelmann, 1904; Howell, 1906). To Harvey we owe not only the discovery of the circulation of the blood, but the first true ideas of the action of the heart, for he showed that