Page images
PDF
EPUB

meshed irregular network; for under a high magnification some of the meshes were found to be formed of a single fibrilla, and it is absolutely impossible to say where one fiber ends and another begins. Certain fibrils from this network end in the sensory spots of the epithelium (Fig. 2, 6, 6'); others terminate freely in the regions between the sensory organs.

There is thus a diffuse sensory nerve supply throughout the epithelium of the palate.

From certain preparations in which the fibrils of this network were incompletely stained, I was able to trace a single fibrilla from one medullated fiber into another without loss of continuity (Fig. 3). This connecting fibril appears to be homogeneous in structure throughout its entire length. It is very possible that such a condition may be produced by the overlapping of two fibrils. I myself have concluded after a careful study of these networks that they are formed by such an overlapping of two fibrils and not by the direct union of fibrils from different "neurones.” In either case it is impossible to say that the fibril belongs to one neurone or to the other.

The presence of fibrillar networks throughout the integument is what we should expect from the physiological facts as to tactile sensations. The whole surface of the skin is more or less sensitive to tactile stimuli but the localization and acuteness of the sensations depend upon the presence of special recep

If two stimuli are applied to a region between sensory spots only one sensation is felt, as the stimulus is diffused and affects equally a number of neurones. If, however, the two stimuli are so far apart as to affect different sensory spots, innervated by distinct nerve fibers, each of these will be strongly stimulated, and two distinct sensations will be the result.

tive organs.

two

Fig. 2. A subepithelial network of neurofibrillae from the palate of the frog; a, a', strands of fibrillae from medullated fibers; b, V, fibrillae which apparently end in the sensory spots; (, é', sensory spots. The network is viewed from the epithelial side of the palate, and the terminations of the sensory fibrils among the cells of the epithelium are indicated by knob-like enlargements. X 3.30; details with LEITZ 1-12 oil immersion.

In addition to this meshwork of sensory fibers there are present in the palate of the frog networks of cells and non

Fig. 3. A portion of a network similar to that shown in figure 2, with two medullated fibers which are apparently directly connected by a fibril. X 1000. medullated fibers; these have been observed by BETHE ('95) and are probably identical with the structures described by DOGIEL ('98) and LEONTOWITSCH (:01). For convenience of description a perivascular and subepithelial network will be distinguished. The perivascular network lies deep in the tissues of the palate and extends wherever blood vessels are abundant. Its meshes are large and the cells comparatively few in number except about the arteries; here a close nework is formed by the fibers and many cells are present (Fig. 4). I have observed

Fig. 4. A nerve network about the walls of an artery (frog); a medullated fiber is seen to be connected with this network. x 380.

similar structures in the floor of the mouth, in the upper part of the oesophagus, and in the wall of the intestine. BETHE states that they may be found in all parts of the integument. In many cases medullated fibers may be directly connected with the mesh-work about the arteries. A connection of this kind is shown in figure 4.

I have never been able to make out special endings in the perivascular networks. The nerve fibrils may often be observed in contact with the circular muscle fibers of the arteries, but no end organs were ever seen at these points.

The network which I have designated as subepithelial is found, as its name implies, directly beneath the epithelium It consists of a rather fine-meshed plexus of cells and non-medullated fibers; a portion of this network is shown in figure 5.

a

Fig. 5. A subepithelial network of cells and non-medullated fibers from the palate of the frog; the network is continuous with a medullated fiber. X 380. The fibers seem to radiate from beneath the sensory organs, and at these points from 3 to 6 cells are usually found grouped together (Fig. 11, a, a'). Into the sensory organs fibers pass from the network, while others are given off at points between

the sensory spots, and end freely in the epithelium. I have never observed fibers from this network innervating the capillaries, but such may be the case. It is, however, connected with the perivascular plexus; medullated fibers also frequently unite with it as may be seen in figure 5. The true nature of these structures has been called into question by many neurologists; I may state here that when I first saw them in my preparations I did not believe that they were nervous structures. After studying them carefully, however, I gathered the following evidence: (1) specific stains for elastic fibers and connective tissue do not demonstrate these networks ; (2) as already observed, the fibers of the networks are continuous with branches from medullated fibers; (3) neurofibrillae may be distinctly observed in well differentiated preparations of these networks and the fibers have the varicose appearance characteristic of nerves. In the face of

Fig. 6. A group of three cells from the subepithelial nerve network, showing the course of the neurofibrillae, and the connection of a large fibril with a medullated fiber; the fibril branches close to the first nucleus, but no network is formed in any of the cells by the neurofibrillae. X 700. these facts there can be no doubt that the fibers of both the perivascular and subepithelial networks are nerve fibers.

As to the nature of their cells, the histological evidence is no means conclusive. My preparations of the frog's palate do not sustain the observations of Bethe (:03) as to the presence of a neuro.

« PreviousContinue »