The cylinder axis is a flexible rod of azotized matter which fills the tube formed by the white medullary tissue around it. When the nerve is torn, the axis cylinder can sometimes be seen projecting, button-like, from the torn end of the tubular membrane and medullary substance. Numerous conjectures have been formed as to the ultimate structure of this portion of the nerve tubule; some having described it as composed of minute longitudinal fibres, while others regard it as made up of laminæ placed one upon another. Every physiological reason would teach us, however, to accept with Schultze, in Stricker's "Handbuch," the belief that the ultimate anatomical neural element is what he terms the primitive fibril, of which a number, great or small, is needed to make an axis cylinder, so that the primitive fibril is the essential nerve element around which, or around groups of which, may be the medullary sheath and the sheath of Schwann,although of the modifications in physiological function which these impress upon it we know absolutely nothing. Fromman, and more recently Grandry,* by exposing the nerves to a peculiar treatment with nitrate of silver, reached the conclusion that the cylinder axis is composed of disks superimposed and isolated by a substance differing from them in composition. I have verified these observations on the sciatic nerve of the rabbit with preparations made by Dr. Keen, and obtained the same result, so that it seems difficult, considering the regularity of the structure thus brought out, to reach any other conclusion than that the axis is probably less simple in construction than has been believed. If we admit with Schultzet and Stricker that this substance also possesses longitudinal striæ, the likeness to the anatomical disposition of the

* Journ. de l'Anat. et Phys., 1869, p. 289.

† Disc. Acad. Bonn, Aug. 1868.

muscle would be remarkable,-a likeness, I may add, for which there may also be some physiological foundation. The mode in which the nerve tubule relates itself to the centres and to the exterior organs is full of interest. It appears now to be pretty generally admitted that the tubule may finally be traced into the cells of the ganglia composing the spinal medulla and the brain, and that before joining these cells it loses its sheath and white covering, becoming thus reduced to the single element of the axis cylinder. It has been inferred from this that the latter part is the essential portion of the ultimate tubule, and that the exterior portions are merely meant to serve for protection or for insulation.

Observers are also well agreed that a similar loss of the external medulla and sheath, and a like thinning of the axis cylinder, is usually observable in the peripheral extremities of nerve tubules; but the most extreme diversity of opinion is held as regards the manner in which they terminate and as to the relation they bear to muscle, on the one hand, and to the sensitive surfaces on the other.

Perhaps no questions in micrology have been less distinctly answered than these. The mass of observers agree that the sensitive nerves may be traced for the most part to a plexiform series of loops which underlie the skin and other sentient surfaces. According to Beale and some others, these plexiform series constitute the true peripheral termination of many sensitive nerves, which, returning again to their central cell connections, form, as it were, a neural circuit. On the extreme outer loops, Dr. Beale located little masses of germinal matter, which he presumes to have an office connected with the incessant maintenance and increase of these ultimate loops,―a view which has been much controverted.

The appearances so described have of late been other

He considers "that

wise interpreted by Langerhaus.* processes of non-medullated nerve fibres from the cutis penetrate between the cells of the rete Malpighii, exactly in the way described by Hager and Cohnheim as the mode of termination of nerves in the cornea. These nerve fibrils pass again into small cells lying between the deeper cells of the rete mucosum, whence fine fibrous outrunners enter the upper layers, to terminate finally in slightly clubbed extremities just beneath the horny layer." These have no relation to tact corpuscles, and the research of Langerhaus, in which Stricker seems to have faith, tends to weaken the belief in terminal peripheral nerve loops for which also physiological ground is wanting.

On the other hand, while it is as yet uncertain whether the sensitive fibres end externally in loops or in absolutely free ends, it is generally held that a vast number are externally related in some way to the little bodies known as the corpuscles of Meissner, of Vater, or Pacini (Vater, Pacinische Körperchen), and of Krause. The latter are found chiefly on mucous surfaces, those of Pacini in the submucous cellular tissue, the mesentery, the muscles, and the papillæ of the derm. These bodies are most numerous in the regions possessed of great tactile sensibility, such as the cushions of the fingers,-M. Meissner having counted eight hundred in a square line of the palmar face of the last phalanx of the index finger.

The structure of these corpuscles does not differ so essentially as to induce the belief that they must have different physiological functions, were it not for their varying anatomical relations to tissues.

The tactile corpuscles of Meissner, for instance, consist of "oblong oval bodies tolerably distinct from the remainder of the digital papillæ in which they lie. They

* Stricker's Comp. Histol., p. 187; New Sydenham Soc. edition.

are generally rounded off at the upper and lower ends, and do not exhibit the longitudinal striation as do the Pacinian bodies, but, on the contrary, transverse nuclei.”* Two nerves can be usually traced to these bodies, but their after-relation to them is less clear. In some cases the nerve seems to envelop the corpuscle spirally, in others, to be lost in the centre of the mass. I have very little doubt that in some instances of local nervous disease the starting-point lies in the dermal nerve papillæ. In a case to which I shall have to refer, the corpuscles of Pacini were certainly both too large and too numerous, and in one which I myself have seen, there was some probability that a neuroma of the thumb was merely an overgrown tact corpuscle.

The corpuscles of Pacini consist of many concentrically arranged layers of connective tissue, always becoming more closely packed towards the centre, and surrounding a cavity filled with soft, abundantly nucleated and easily alterable material, which coagulates after death, and into the interior of which the nerve fibres penetrate. These, after they have lost the medullary sheath and the sheath of Schwann,-which latter becomes continuous with the laminated sheath of connective tissue investing the corpuscle, consist only of the axis cylinder, which terminates in a little bulb.

The nerve corpuscles of Krause, described and depicted by him as existing in the conjunctiva, genitals, and other mucous surfaces, differ from the Pacinian corpuscles only by the absence of a thick, laminated investment.†

Most authors have held that these little bodies are apparatuses of reinforcement (Vulpian) for the impressions

*Virchow, Cellular Pathology, translated by Dr. Chance, p. 277. New York, 1861.

† Schultze, in Stricker's Histology, New Syd. Soc. Transactions, p. 168.

to which the sensitive nerves are submitted, or that each corpuscle is a centre of ganglionic matter, without which certain impressions cannot originate,-a view sustained, to some extent, by the analogy of some of the special senses, but contradicted by a host of pathological phenomena. Their function may possibly be protective, as regards the nerve ends, but that they have some relation to general sensibility, or to touch, seems alone clear. Rauber-who has stated the number of the deep-seated corpuscles of Pacini at 2142, too small a number for large relations to sensation-believes them to be the sensory organs as to the muscles. Vulpian states very justly that except as to the well-known mesenteric connection between certain Pacinian bodies and the sympathetic nerve, we have no clear information as to the peripheral distribution of this nerve system, nor are we much better instructed in the mode in which nerves terminate on vessel walls, a subject of daily increasing interest.*

The

Since the researches of M. Rouget and their general confirmation by Krause and Kühne, there is less difference of sentiment as to the motor termination of nerves. nerve fibre undergoes division, and each branch on entering the sarcolemma loses its sheath of Schwann, which becomes continuous with the sarcolemma. The axis cylinder alone enters, the medullary matter as in other cases having previously disappeared. The cylinder axis of the nerve spreads out over the muscle substance in a granular mass, which is slightly prominent, and as to the true nature of which observers have disagreed. According to Trinchese, and to Rouget's latest researches, the granular cone, now known as the motor plate, conceals a delicate set of fine loops or plexuses, which are the true terminations of the motor nerve. Dr. Beale, however, believes that

* See Duchenne, translated by H. Tibbits, M.D. London, 1871, p. 153.