6o THE POPULAR SCIENCE MONTHLY 



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 pupillae of the eye in vertebrates and the heart-muscle 

 may be considered. The sphincter pupillae is a ring of muscle im- 

 bedded in the iris and surrounding the pupil in the eyes of most verte- 

 brates. 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 

 adjuster from which the oculomotor nerve transmits peripherally to the 

 effector, the sphincter pupillse. In the lower vertebrates, particularly 

 in the fishes and amphibians, it has long been known that the sphincter 

 pupillse 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 

 pupillse of the vertebrate eye, though usually controlled by nerves, is 

 a muscle that can be directly stimulated and in this respect is an inde- 

 pendent 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 



