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indeed, have been published which show this, and much else; but such figures are diagrams, not copies of what is seen. They belong to Imaginary Anatomy. * The relation of the cell-process to the nerve-fibre will be discussed anon.

125. A word in passing on the contradictory assertions respecting the anastomosis of nerve-cells. That the gray

a

Fig. 19.

Anastomosing nerve-cells (after Gratiolet). a, body of the cell; c, process

of uniting two cells; d, branching process.

substance forms a continuum of some kind is certain from the continuity of propagation of a stimulus. But it is by no means certain that one cell is directly united to its neighbor by a cell-process. Eminent authorities assert that such direct union never takes place; others, that it is a rare and insignificant fact; others, that it is constant, and "demanded by physiological postulates." I will not,

* When men of such experience and skill as KÖLLIKER, BIDDER, GOLL, and LOCKHART CLARKE declare that they have never seen a cellprocess pass directly into a dark-bordered fibre in the anterior root, what are we to say to such figures and descriptions as those given in the works of SCHRÖDER VAN DER KOLK, GRATIOLET, and Luys? Even did such arrangements exist, no transverse nor longitudinal section could display them, owing to the different planes at which the fibres enter, and the length and irregularity of their course.

in the presence of distinct affirmations, venture to deny that such appearances as are presented in Fig. 19 may occasionally be observed; the more so as I have myself seen perhaps half a dozen somewhat similar cases; but it is the opinion of Deiters and Kölliker that all such appearances are illusory.* Granting that such connections occur, we cannot grant this to be the normal mode; especially now the more probable supposition is that the connection is normally established by means of the delicate ramifications of the branching processes.

Imaginary Anatomy has not been content with the cells of the anterior horn being thus united together, to admit of united action, but has gone further, and supposed that the cells of the posterior horn, besides being thus united, send off processes which unite them with the cells of the anterior horn — and thus a pathway is formed for the transmission of a sensory impression, and its conversion into a motor impulse. What will the reader say when informed that not only has no eye ever beheld such a pathway, but that the first step- the direct union of the sensory nerve-fibre with a cell in the posterior horn-is confessedly not visible?

126. The foregoing criticisms will perhaps disturb the reader who has been accustomed to theorize on the data given in text-books; but he may henceforward be more cautious in accepting such data as premises for deduction, and will look with suspicion on the many theories which have arisen on so unstable a basis. When we reflect how completely the modern views of the nervous system, and the physiological, pathological, and psychological explanations based on these views, are dominated by the cur

* Long after the text was written, WILLIGK published in Virchow's Archiv, 1875, LXIV. p. 163, observations of anastomoses which even KÖLLIKER admitted to be undeniable. Yet out of sixty-four preparations, amid hundreds of cells, he could only reckon seven cases of conjunction.

rent notions of the nerve-cell, it is of the last importance that we should fairly face the fact that at present our knowledge even of the structure of the nerve-cell is extremely imperfect; and our knowledge of the part it plays - its anatomical relations and its functional relations is little more than guesswork!

THE NERVES.

127. We now pass to the second order of organites; and here our exposition will be less troubled by hesitations, for although there is still much to be learned about the structure and connections of the nerve-fibres, there is also a solid foundation of accurate knowledge.

A nerve is a bundle of fibres within a membranous envelope supplied with blood-vessels. Each fibre has also its separate sheath, having annular constrictions at various intervals. It is more correctly named by many French anatomists a nerve-tube rather than a nerve-fibre; but if we continue to use the term fibre, we must reserve it for those organites which have a membranous sheath, and thereby distinguish it from the more delicate fibril which has none.

The nerve tube or fibre is thus constituted: within the sheath lies a central band of neuroplasm identical with. the neuroplasm of nerve-cells, and known as the axis cylinder; surrounding this band is an envelope of whitish substance, variously styled myeline, medullary sheath, and white substance of Schwann: it is closely similar to the chief constituent of the yolk of egg, and to its presence is due the whitish color of the fibres, which in its absence are grayish. The axis cylinder must be understood as the primary and essential element, because not only are there nerve-fibrils destitute both of sheath and myeline yet fulfilling the office of Neurility, but at their termi

[graphic]

Fig. 20.-a, axis cylinder formed by the fibrils of the cell contents, and at a assuming

nations, both in centres and in muscles, the nerve - fibres always lose sheath and myeline, to preserve only the neuroplasmic threads of which the axis cylinder is said to be composed. In the lowest fishes, in the invertebrates, and in the socalled sympathetic fibres of vertebrates, there is either no myeline, or it is not separated from the neuroplasm.

128. Nerve-fibres are of two kinds -1°. The dark-bordered or medullary fibres, which have both sheath and myeline, as in the peripheral system; or only myeline, without the sheath, as in the central system. 2°. The non-medullary fibres, which have the sheath, without appreciable myeline-such are the fibres of the olfactory, and the pale fibres of the sympathetic.

Nerve-fibrils are neuroplasmic threads of extreme delicacy, visible only under high magnifying powers (700-800), which abound in the centres, where they form networks. The fibrils also form the terminations of the fibres. Many fibrils are supposed to be condensed in one axis cylinder. This is represented by Max Schultze in Figs. 17 and 20.

129. As may readily be imag

the medullary sheath; b, naked ined, the semi-liquid nature of the

axis cylinder from spinal cord.

neuroplasm throws almost insuperable difficulties in the way of accurately determining whether the axis cylinder in the living nerve is fibrillated or not; whether, indeed, any of the aspects it presents in our preparations are normal. Authorities are not even agreed as to whether it is a pre-existent solid band of homogeneous substance, or a bundle of primitive fibrils, or a product of coagulation.* Rudanowsky's observations on frozen nerves convinced him that the cylinder is a tubule with liquid contents. My own investigations of the nerves of insects and molluscs incline me to the view of Dr. Schmidt of New Orleans, namely, that the cylinder axis consists of minute granules arranged in rows and united by a homogeneous interfibrillar substance, thus forming a bundle of granular fibrils enclosed in a delicate sheath in other words, a streak of neuroplasm which has a fibrillar disposition of its granules. We ought to expect great varieties in such streaks of neuroplasm; and it is quite conceivable that in the Rays and the Torpedo there are axis cylinders which are single fibrils, and others which are bundles, with finely granulated interfibrillar substance.§

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The fibres often present a varicose aspect, as represented in Fig. 21. It is, however, so rarely observed in the fresh tissue, that many writers regard it (as well as the double contour) as the product of preparation. It is, indeed, always visible after the application of water.

See the history given in STILLING's learned work, Ueber den Bau der Nervenprimitiv-Faser, p. 34; and compare MAX SCHULTZE, De Retina Structura, p. 8, and Bau der Nasenschleimhaut, p. 66; WALDEYER, in the Zeitschrift für rat. Med., 1863; LISTER and TURNER, Observations on the Structure of Nerve-Fibres, in Quarterly Micros. Journal, 1859; RANVIER, in the Archives de Physiologie, 1872.

+ Virchow's Archiv, Bd. LXXII. p. 193.

Monthly Journal of Micros. Science, 1874, XI. p. 214.

§ BABUCHIN, Centralblatt, 1868, p. 756.

Even so eminent an authority as W. KRAUSE holds this both with

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