It is no longer necessary to demonstrate that the nerve activity is evidenced by increased metabolism and raising of the temperature. Visible changes in the nerve cells after intense stimulation of a nerve have been demonstrated. One is scarcely able to decide whether the chemical process which takes place in nerve activity, as such represents the nervous conduction of the stimulus (neurokyme), or whether it produces more physical molecular wave movements. It is possible that in the mysteries of the molecular processes of organic life the chemical and the physical are not always capable of being so sharply differentiated.

We are justified in placing the processes which we call inhibition and its reverse, increasing of stimulation and opening up new paths (Bahnung of Exner), in the substance of the ganglion cells and in the terminal branchlets or clubs of the neurones - that is, in those portions of each bordering on the other.

Certain anatomical facta appear to me to be important. The phenomena of memory appear to exclude the possibility of a destruction of brain elements, and a substitution of the same by new elements in the course of the post-embryonic life. This question caused me to have the matter investigated, and I therefore directed Dr. Schiller (at that time my assistant in Burghoelzli, and now Director in Wyl) to determine whether the number of the elements in the central nervous system increased after birth or not. According to his results, it appears that the number does not increase in the oculo-motor nerve of the cat, but that the size of the elements does increase.1 It is therefore extremely probable that the same nerve elements persist during the whole post-embryonic life. Birge had already shown that the number of ganglion cells in the motor nerve nuclei of the frog corresponds to the number of fibers. Pathological foci in the brain, and also the results of Gudden's brain operations on animals, prove that the brain elements, once they are destroyed, cannot be formed afresh. Only the axis cylinder of peripheral nerves can grow again through the nodes of Ran-vier as long as the corresponding ganglion cell is intact.

His and I attempted to prove independently of one another, in 1886-1887, the indivisibility of nerve elements by means of important facts.2 Basing an opinion on the embryonic growth of fibers from cells (His), and on the dependence of the fiber on the cell and the cell on the fiber in pathology and in experimental research (Forel), we denied the occurrence of anastomosis, and claimed that each fiber belongs to its own cell, existing in the form of a process. Our views have been confirmed later by Ramon y Cajal and Koelliker histologically. Waldeyer gave the nerve element (cell with its dependent branching fibers) the name of neuron, and the whole was termed the neurone theory. This agrees quite well with Schiller's results.

Nissl then studied the structure of the ganglion cells more closely by means of staining methods, and Apathy demonstrated especially the fibrilla both in the sheathlesa nerve fibers of inter-vertebrate animals and in ganglion cells with the help of excellent staining. The last-named undoubtedly proved the existence of fibrilla anastomosis in the protoplasm of the ganglion cells of the leech. Apathy therefore considers that the neurone theory can be refused, since he propounds the theory that ganglion cells are not nerve cells, but are only traversed by fibrilla. The fibrilla are supposed to be the product of other cells, which he calls nerve cells, and which are distributed everywhere, even in the white substance. He returns to Gerlach's fiber network. In his opinion, the fibrillum is the nerve element, and is anatomically present everywhere in the gray as well as in the white substance. He considers that the cells of Schwann's sheath and the corresponding cells of the neuroglia ("intermediate substance," regarded previously as connective or epithelial tissue, and not as nervous) are derived from the nerve fibrilla. He therefore calls them fibrillogenous nerve cells. These fibrillo-genous nerve cells would thus continuously be able to form new fibrilla and new anastomoses, even in the central nervous system.

1 Comptes Rendus de l'Acad. des Sciences, September 30. 1889. The size of libera of the adult cat is six to eight times that of fibers of the new-born

2 His, "The Human Spinal Cord and Nerve Roots"; and Forel, "Observations on the Anatomy of the Brain and their Results," (Arch. f. Psychiatrie).

One does not dispute, and has never disputed, that a new formation of peripheral nerve elements and of nerve elements of lower animals takes place. Without this assumption it would be impossible for the amputated tail of a lizard to regenerate. But, on the other hand, Apathy's theory does not agree with a number of important facts, and the physiological experiments carried out by Bethe, on which Apathy bases his opinions, do not deserve any consideration, since Bethe has revealed his suspicious unreliability in dealing with other subjects. Still, Apathy's results and views were warmly welcomed, for they led to a profounder investigation of the question. The later works of Ramon y Cajal, Wolff, Harrison, and others, have disproved Apathy's views. Harrison has shown that peripheral motor nerves grow solely from the cells of the anterior horns after destruction of the embryonic site of the sheath of Schwann.

Matthias Duval, on the other hand, has exaggerated the neurone theory by presuming that the terminal treelets of the branchings of the fibers of a neuron are possessed with amoeboid movement. He attempts thereby to explain not only sleep (through the retraction of the pseudopodia and breaking off of contact), but also of inhibition and conduction of stimuli. Wiedcrsheim is supposed to have observed something of a similar nature in transparent animals. But, for all that, it appears to me that the whole subject encroaches on the territory of hypothetical speculation.