Page:Popular Science Monthly Volume 75.djvu/146

142 cular muscles on which rests a second nervous layer, Meissner's plexus. Each plexus, so far as is known, is a true nervous net as intimately related to the adjacent muscle fibers as is the case of the sea-anemones. In fact one of the muscle layers and the adjacent plexus in the intestine reproduce very accurately all the essentials of the neuromuscular mechanism of a sea-anemone except the epithelial sense-cells.

Not only is there this anatomical similarity between the neuromuscular mechanisms of the sea-anemone and of the vertebrate intestine, but there is also a physiological similarity as seen in the movements of the digestive tube. The essentials of these movements are well exemplified in the small intestine. In this part of the digestive tube the characteristic movements are segmentation and peristalsis. Segmentation consists in a series of temporary, ring-like constrictions in the intestinal wall that come and go in such a way that the enlarged region of the tube between any two constrictions is the site of the constriction next to appear, and so on. As a result of segmentation, the food is most thoroughly churned and mixed. Peristalsis is a wave-like movement whereby the food is carried posteriorly through the intestine. Usually these two movements go on together in such a way that the peristalsis is combined with segmentation in that the latter becomes somewhat unsymmetrical and cuts each food mass into two unequal parts the larger of which is on the posterior side of the constriction. Hence the food is not only churned but is at the same time moved posteriorly through the intestine.

The small intestine receives nerve-fibers from two extraneous sources, the vagus and the splanchnic nerves, and it might be supposed that these were essential for the movements of the intestine. But as Cannon (1906) has demonstrated, both sets of nerves may be cut, and yet after recovery from the immediate effects of the operation segmentation and peristalsis will be found to go on in the digestive tube in an essentially natural manner. It is thus clear that the vertebrate intestine, like the tentacle of a sea-anemone, contains a complete neuromuscular mechanism within its own wall, and though there is no histological evidence of the presence of receptors reaching from the mucous surfaces of the intestine to the nervous nets within, yet there are sound physiological grounds for assuming the presence of such organs. In that case the type of neuromuscular mechanism in the intestine would be practically identical with that in the sea-anemone.

A second example of a receptor-effector system in ccelenterates is seen in the jellyfishes. In these animals as contrasted with the seaanemones, locomotion is a well-developed activity, and it is the neuromuscular mechanism concerned with this function that must be considered. The structures involved in locomotion are well exemplified in Aurelia (Fig. 4). This common jellyfish possesses on the edge of