G. H. Parker

“ 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. ”

“ The feeding habits of the sea-anemones throw considerable light on the physiology of their nervous structures. If particles of meat are dropped on the tentacles of an expanded Metridium, they become entangled in the mucus on these organs and are quickly delivered to the mouth, where they are swallowed. If fragments of clean filter-paper soaked in sea water are similarly dropped on the tentacles, they are usually discharged from the edge of the oral disk without having been brought to the mouth. Thus the animal appears to discriminate between what is good for food and what is not. ”

“ 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. ”

“ The first neurone, as exemplified in the nervous structure of such an animal as the earthworm, is often the body of a sense-cell on the surface of the animal and the sensory nerve-fiber to which this cell body gives rise and which leads to the central nervous organ. The second neurone is a nerve-cell whose body lies within the central nervous organ and whose process, a motor nerve-fiber, extends from the central organ to the muscle-cells which it controls. ”

“ But the association neurones are not only the most numerous in the vertebrates; they also constitute the basis of the most significant evolution. The central nervous organs that show the most conspicuous progressive changes in the vertebrates are the cerebellum and the cerebrum, particularly their cortical portions, and when it is remembered that few or no primary sensory or motor neurones contribute to these two organs, but that they are made up of association neurones almost exclusively, it will be seen how enormously important these neurones become. ”

“ In a similar way the adjustor or central organ is useless without at least some other element in the reflex apparatus. The only mechanism sufficient in itself is the effector, which, if it can be brought into action by direct stimulation, may accomplish something serviceable to the animal. It is therefore improbable that we shall find multicellular animals that possess either receptors or adjusters without effectors, but it is conceivable that primitive metazoans may have effectors without other parts of the typical neuromuscular mechanism. ”

“ Another point of comparison between the nervous mechanism in cœlenterates and in the earthworm is the presence of nerves in the latter and their absence in the former. As already pointed out, the nerves in the earthworm are bundles of independent fibers which course more or less together between their end-organs and the central apparatus. The fibers in a nerve have no necessary functional relations one with another, but are brought together chiefly by convenience of passage. ”

“ At the beginning of each experiment the receptors are stimulated by the strong juices of the meat and the weaker juice of the paper. As they run down in efficiency, they come to a stage where they no longer react to the weaker stimulus of the paper and respond only to the meat. At this stage apparent discrimination takes place.
Not only do these experiments show no evidence of central nervous functions, but they indicate a decided looseness of nervous articulation. The activity of one side of the body of the sea-anemone has very little, if any, influence on the other side. ”

“ For this reason the central nervous system of the vertebrate, though giving much evidence of segmentation in its early stages of growth, is finally a physiological unit such as is realized in no other group of animals, a condition well evidenced by the fact that some of its most recent phylogenetic acquisitions, like the pyramidal tracts of the mammals, may consist of neurones that reach almost from one end of the system to the other. ”

“ If what has been stated in this article is correct, we must picture to ourselves as steps in the evolution of the nervous system not only the independent origin of muscle around which the nervous organs subsequently develop, but also the independent origin of other effectors such as chromatophores, glands and phosphorescent organs and the secondary appropriation of many of these by a developing nervous system. ”

“ If instead of removing the nerves, the median and lateral strands are cut through at any plane, care being taken not to injure the underlying heart-muscle, the two regions of the heart thus established beat independently and coordination of the heart as a whole is lost. If the nervous connections are left intact but the muscular heart is completely cut across in several places, the whole organ continues to beat in complete coordination. ”

“ There is good reason to believe that the cerebral ganglion or brain of the earthworm is in a measure degenerate. Certainly if we turn to such an annelid as Nereis we find in place of the small mass of ganglionic cells and fibers that represent the brain in the earthworm a much more extensive organ connected with a considerable number of sense organs none of which are present in the earthworm. ”

“ This aperture leads inward through a short gullet to a large, somewhat divided, digestive cavity, the gastrovascular space, which extends throughout the whole interior of the animal even to the tips of its tentacles and is the only cavity within the sea-anemone. The body of the animal is made up of walls of extreme thinness; these walls consist of two layers of cells, an outer one next the sea water, the ectoderm, and an inner one next the gastrovascular space, the entoderm. ”

“ The primary motor neurones of vertebrates (Fig. 6) resemble very closely those of invertebrates, for their cell-bodies are within the central nervous mass and their neurites extend as motor nerve-fibers to the skeletal muscles. The primary sensory neurones also agree with those of the invertebrates except that their cell-bodies instead of being in or near the integument, as in most invertebrates, have migrated centrally and thus form the dorsal ganglia. ”

“ The significance of the various parts of the neuromuscular mechanism in such an animal as Aurelia can be determined by experiment. If the eight sense-bodies are removed, the animal will no longer pulse spontaneously, though its muscles may be made to contract by direct stimulation. If all but one sense-body are removed, the bell will pulse with regularity and by artificially stimulating the single remaining body a wave of muscular contraction can be sent over it. ”

“ In the vertebrates at least, nervous effectors include not only muscles, but also electric organs. These organs occur not infrequently among the fishes. They are best represented in the South American electric eel, the electric catfish of Africa and the torpedoes of the Mediterranean Sea and the Atlantic and Indian Oceans. They also occur less fully developed in certain skates, mormyres and the star-gazer. These organs are usually imbedded in a mass of the fish's muscle or they occupy such positions that they clearly replace muscles. ”

“ The inward and outward migration of the pigment of the chromatophores is well seen in the skin of the so-called Florida chameleon, Anolis. According to Carlton (1903) , who has studied this animal with care, the passive state in its chromatophores is that in which their pigment is gathered together in the cell-bodies. This state is brought about when the lizard is removed from the stimulating effect of light, when the blood and nerve supply of a given region are cut off, when the animal is etherized, or when it dies. ”

“ The heart itself is a long, segmented, muscular tube situated near the dorsal line of the animal. On the dorsal face of the heart is a median nerve-cord containing ganglion-cells and connected with two parallel lateral nerve-strands that lie near the sides of the heart. This whole nervous mechanism may be dissected off from the heart, leaving this organ in other respects intact. ”

“ In this way the central nervous organs of these animals are locked together much more closely than are those in the worm and exhibit consequently in their physiology a unity that the worms do not possess. This nervous unity, moreover, has developed to such a degree in the higher arthropods that we may with reason ascribe to such animals as the insects a primitive form of intellectual life not unlike that found in the vertebrates. ”

“ They are characteristic of those animals in which sense organs and muscles have become well differentiated and widely separated from the central organs, and are not to be confused with elongated bundles of nervous elements such as are to be met with in some cœlenterates and many echinoderms, for though these may represent early steps in the evolution of nerves, they still retain so many evidences of functional interrelation among their elements that they are to be classed rather with nervous nets than with nerves. ”

“ If what has been said in these lectures is true, namely, that sense-organs in general precede central nervous organs in evolution, then the brain of the worm has developed at its anterior end because the chief sense-organs were originally there, and not vice versa, a statement that I believe to hold for the growth of the brain in all animals. ”

“ Although most glands are not under direct nervous control, some are as completely under this control as the majority of muscles are. The best examples of this condition are the sweat glands and the salivary glands. The fact that when the nerves supplied to the salivary gland are stimulated, secretion may take place at a pressure higher than that of the blood supplied to the gland shows conclusively that the production of saliva is not a simple organic filtration process, but is dependent upon action called forth in the secretory cells by a nervous impulse. ”

“ There is good reason to assume that the heartbeat in tunicates is of myogenic origin and the fact that the embryonic vertebrate heart pulses before it contains any nervous elements is strong evidence in favor of the view that the cardiac muscle of the primitive vertebrate was a muscle developed independently of nervous control. ”

“ From this general discussion it is quite evident that the cardiac muscles of different animals act in very different ways and that while some, like the heart of Limulus, have a neurogenic beat, others like that of the tunicates have a myogenic beat. ”

“ That that muscle in modern adult vertebrates is under a certain amount of nervous control is unquestionable, but this control is not of the kind usually seen in other neuromuscular combinations. The nerves that enter the heart are probably not ordinarily directly concerned with its beat, for, as already pointed out, this continues after they are cut. ”

“ If now the median nerve-cord and the lateral strands are dissected away, the heart comes to a standstill and never again shows a natural beat, though a stimulus applied directly to its substance will cause it to contract. ”

“ From this standpoint these cells are true chromatophores. In an eye exposed to the light the pigment-granules occupy distal positions in these cells; in one in the dark they come to lie in proximal positions. The place occupied by the pigment in a given eye is entirely determined by the presence or absence of light in that eye, for the two eyes have no sympathetic relations. Moreover if a persistent shadow is cast on part of one eye, the condition characteristic for the dark is assumed by that part even though the pigment in the rest of the eye is in the position characteristic for light. ”

“ When the bell of an Aurelia is pulsing, the movement is carried out by the more or less general contraction of this circular band of muscle, which is brought back to its original position on relaxation by the elasticity of the gelatinous mass of the bell. The locomotor muscle, then, is a gigantic sphincter that works against an elastic resistance. ”

“ This line of progressive differentiation from the muscle to the complete nervous system is complicated by the fact that in the more complete examples of the third stage the nervous system is found connected not only with such effectors as muscles, but with electric organs, chromatophores, glands, luminous organs, etc. ”

“ Physiologically as well as anatomically the sea-anemone seems to possess a diffuse rather than a centralized nervous system, and its neuromuscular mechanism consists of receptors and effectors connected by a nervous net which is composed partly of the nervous processes of the receptor cells and partly of similar processes from ganglion cells. ”

“ Whether the gray substance of the central organs in these animals contains a true nervous net as seems to be the case in many invertebrates or whether their neurones retain greater individuality and are related morphologically only through contact, is not yet settled. ”

“ It is to be noted that every reflex mechanism is in the nature of a physiologically continuous span of living substance which reaches from the receptive surface on the one hand to the effector organ on the other. ”