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In the nervous system of vertebrates networks of cells and fibers have been studied chiefly in connection with the blood vascular system. They have been described by DOGIEL ('93, '98), LEONTOWITSCH (:01), CAVALIÉ (:02), BETHE ('95, :03) and others. BETHE states that such networks are present throughout the whole integument of the frog. They form a close network about the blood vessels and a wide-meshed subepithelial plexus. DOGIEL ('98) and LEONTOWITSCH (:01) have carefully studied the networks in the human skin. They assert that connections exist between these structures and the medullated fibers, but their figures do not convince one of this. Their statements have, however, been verified by BETHE (:03) who figures a medullated fiber continuous with a wide-meshed sub-epithelial plexus. In the vertebrate heart also, DOGIEL ('98), HofMANN (:02) and BETHE (:03) have observed independently a network of cells and fibers surrounding the muscle bundles. This network resembles closely the diffuse nervous system of the Medusae and BeThe maintains that the structures he has seen are undoubtedly of a nervous nature.

It has been stated even recently by certain investigators, that both the cells and fibers composing these networks are nonnervous structures. BARDEEN (:03), among others, has exexpressed his doubts as to their nervous character; he criticises LEONTOWITSCH and suggests that the whole network described by the latter may be composed entirely of connective tissue. Even if the fibers are nervous structures the cells may be merely sheath cells.

It is important both to the teacher and student of neurology that these doubts be either confirmed or entirely removed. If networks of true nerve cells and fibers really exist in the integument of vertebrates, then the idea that the peripheral nerves originate only from ganglion cells in or near the central nervous system must be abandoned. If, however, the networks are proved to be nothing more nor less than connective tissue structures, the opponents of the neurone theory have lost one of their strongest arguments.

My research was begun in the Physiological Institute of the University of Strassburg, where I was studying as PARKER Fellow of Harvard University. While demonstrating with methylene blue the innervation of the frog's heart, I obtained several interesting preparations of the nervous elements in the palate, which led me to a further investigation of their structure, This investigation has enabled me not only to verify several points which have been hitherto in doubt, but also to observe new structures which other investigators have either overlooked or have failed to demonstrate.

In the present paper I shall first give evidence from preparations of the normal palate to show that the fibers of the networks described are true nervous structures. And, secondly, from degeneration preparations I shall endeavor to show whether the cells present in these networks are sheath cells or are as truly nerve cells as those of the brain and sensory ganglia.

I.

THE NERVOUS STRUCTURES OF THE PALATE.

Preparations were obtained by injecting /2 cc. of a 1% solution of methylene blue (in normal salt solution) into the abdominal vein of the frog. The animals were either rendered passive by the subcutaneous injection of curare, or tied out immovable on the wooden frame shown in figure 8, p. 107. Within five or ten minutes after the appearance of the stain in the integument, the palate with its nerves and vessels was dissected from the roof of the mouth-an easy task, thanks to the lymphsinus lying beneath the integument. The preparation was then placed epithelial side down, in a flat watch crystal and the exposed surface moistened with the animal's blood while the progress of the stain was watched under the microscope. When the right degree of staining was judged to be obtained, the blood and mucus were rinsed away with normal salt solution and the tissue fixed with ammonium picrate. The preparations were first usually mounted in glycerin, studied in the fresh condition, and important details sketched with the camera lucida. They could then be quickly washed in water, refixed in ammonium molybdate and mounted in balsam. The molybdate method gives much clearer mounts, but has this disadvantage, that the finer details are often lost by the washing out of the stain in running the preparations up through the alcohols. By studying preparations by both methods I thus did away with the disadvantages of each. Many mounts were made between two cover glasses, allowing the use of an oil immersion from both sides. This is a distinct advantage when whole mounts are employed.

The frog's palate is innervated chiefly if not exclusively by the Ramus palatinus of the seventh cranial nerve (facialis). Each palatine branch (Fig. 1) passes down to the roof of the

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Fig. 1. The roof of the frog's mouth with the integument dissected away to show the course of the palatine nerves. R. palatinus, palatine nerve; R. comm. V. 2, Ramus communicans of the trigeminal nerve; a, b, points at which nerve was severed. (After GAUPP). mouth immediately anterior to the lateral process of the basisphenoid, runs nearly straight cephalad and then, bending sharply laterad, joins the “Ramus communicans of the trigeminus. Along its course the nerve gives off many lateral twigs, the fibers of which interlace to form an intricate plexus of medul

lated fiber-bundles over the whole inner surface of the palate. From this plexus fibers pass off to the epithelium. According to BETHE each medullated fiber divides into four branches and each branch innervates a different sensory organ, the number for each 'organ being but two. This peculiar and definite method of innervation, he points out, is the most natural arrangement by which each end organ may receive a distinct nerve supply at the expense of the smallest number of nerve fibers—a separate nerve supply for each sensory organ being requisite for the localization of tactile stimuli.

My preparations confirm the general conclusions of BETHE, but the distribution of the sensory fibers is not as simple as he supposed. It is true that usually only two or three large fibers innervate each sensory organ; these break up into numerous fine fibrils, which, after a tortuous course, end between the epithelial cells. The sensory organs in which these fibers end project slightly above the surface of the palate and are most numerous at the sides. BETHE counted an average of 210 end organs but only 70 fibers in the palatine nerve; if each fiber branches into four, as BETHE asserts, this would allow an average of between two and three branches for each sensory spot. But in addition to these branches I find numerous bundles of fibrillae given off from each medullated fiber. These divide into still smaller fibrils which form a network of fine neurofibrillae and probably connect the different sensory organs. This network has not to my knowledge been observed in the integument of the frog, but SFAMENI (:02) and RUFFINI (:01) have recently described structures apparently identical to it in the skin of man. In the frog the fibrillae composing the network are very difficult of demonstration. In the great majority of methylene blue preparations they are but incompletely stained, and of good preparations I obtained but two or three out of perhaps a hundred trials. The network lies directly beneath the epithelium and is composed entirely of non-medullated fibrillae (Fig. 2). Strands of these are given off from the medullated fibers as seen in the figure at a and a'. The strands divide and their fibrils are apparently continuous with each other in a fine

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