TERMINATION OF NERVES.

cxlix

but the sheath does not long retain the densely fibrous character of the membrane with which it is thus connected at its commencement.

The arrangement of the membranes on the roots of certain of the cranial nerves requires to be specially noticed.

The numerous fasciculi of the olfactory nerve pass through their foramina almost immediately after springing from the olfactory bulb, and then also receive their neurilemma. The bulb itself, and intracranial part of the nerve, which are to be regarded as being really a prolongation or lobe of the brain, are invested externally by the pia mater, but are not fasciculated. The arachnoid membrane passes over the furrow of the brain in which this part of the nerve lies, without affording it a special investment.

The optic nerve becomes subdivided internally into longitudinal fasciculi by neurilemma a little way in front of the commissure: on passing through the optic foramen it receives a sheath of dura mater, which accompanies it as far as the eyeball. The acoustic nerve becomes fasciculated, receives its neurilemma, and acquires a firm structure on entering the meatus auditorus internus in the temporal bone, towards the bottom of which it presents one or more small ganglionic swellings containing the characteristic cells. Up to this point it is destitute of neurilemma, and of soft consistence, whence the name "portio mollis" applied to it.

The larger root of the fifth pair acquires its neurilemma and its fasciculated character sooner at its circumference than in the centre, so that, in the round bunch of cords of which it consists, those placed more outwardly are longer than those within, and, when all are pulled away, the non-fascicular part of the nerve remains in form of a small conical eminence of comparatively soft nervous substance.

Most of the nerves have ganglia connected with their roots. Thus, the spinal nerves have each a ganglion on the posterior of the two roots by which they arise; and in like manner several of the cranial, viz., the fifth, seventh, glosso-pharyngeal, and pneumo-gastric, are furnished at their roots, or at least within a short distance of their origin, with ganglia which involve a greater or less number of their fibres, as described elsewhere in the special anatomy of those nerves.

Termination, or peripheral distribution, of nerves. It may be stated, generally, and apart from what may apply to special modes of termination, that, in approaching their final distribution, the fibres of nerves, medullated and non-medullated, commonly divide into branches (fig. LXXXV); and the former, either before or after division, generally lose their medullary sheath, and consequently their dark borders, and take on the characters of pale fibres. The axis-cylinder participates in the division, and it might be said that the white fibres are represented in their further progress, by the axis-cylinder and its ramifications; still, the primitive sheath or membranous tube continues some way along these pale branches after the medullary sheath has ceased, but may finally too desert them. By repeated division the fibres become smaller and smaller; but whilst some of the resulting small fibres may be simple, many are really bundles of exquisitely fine pale fibrils, straight, sinuous, or somewhat tortuous in their course. They bear nuclei, some of which, no doubt, may appertain to the prolongation of the primitive sheath; but others, generally fusiform and granular, are interposed, as it were, in the course of the fibres, and are continuous with them at either end; nuclei, moreover, of a triangular or irregular shape, are common at the bifurcations of the fibres. These pale fibres often join into networks; but their further disposition in different parts will be treated of below. In the meantime it must be explained that the original dark-bordered fibres which thus undergo division and change, or which may proceed singly to end in a different and special manner, are commonly provided with a tolerably strong sheath with nuclei, which, as it stands well apart from

the dark borders of the fibre, is very conspicuous. This is sometimes considered to be only the primitive sheath of the fibre modified in character,

Fig. LXXXV.-SMALL BRANCH OF A MUSCULAR NERVE OF THE FROG, NEAR ITS TERMINATION, SHOWING DIVISIONS OF THE FIBRES.

a, into two; b, into three; magnified 350 diameters.-(From Kölliker.)

but it seems more probable that it is derived from the neurilemma or perineurium which incloses the fine bundles or funiculi, and, as these part into smaller collections and single fibres, undergoes a corresponding division, and finally sends sheaths along single fibres.

In further treating of the terminations of nerves it will be convenient to consider the sensory and motor nerves separately.

Of the sensory, or, at least, non-muscular nerves, the following modes of final distribution have been recognised.

A. By networks, or terminal plexuses. These are formed by the branching and interjunction of the pale fibres above described. The meshes of the net may be at first wider, and the threads, or bundles of threads, larger, but from these, finer filaments forming closer reticulations proceed, and then sometimes the nuclei become less frequent, or disappear. Such networks are found in the skin of the frog, rat, and mouse; in various parts of the mucous membranes, in the cornea, and also in the connective tissue beneath serous membranes or between their layers in different parts -of which the mesentery of the frog affords a good example. In some of these cases the nerve-fibres come into the vicinity of connective-tissue-corpuscles, but, so far as I have been able to see, are not connected with them.

B. Sensory terminal organs. Three varieties of these are now recognised, viz., a., end-bulbs-b., touch-corpuscles, and c., Pacinian bodies. These have so far a common structure, that in all of them there is an inward part or core (Innenkolben Germ.) of soft, translucent, finely granular matter; an outer capsule of ordinary connective tissue with its pertaining corpuscles; and, finally, one or sometimes more nervefibres, pale and without dark contours, which pass into the core and apparently end with a free, usually somewhat swollen, or knobbed extremity. Thus agreeing in their internal and probably essential structure, the terminal organs differ chiefly, or at least most obviously, in their capsule, which, simple in the end-bulbs, becomes

highly complicated in the Pacinian bodies; and therefore in the further account of them it will be convenient to begin with the former, although the Pacinian bodies have been much longer known.

a. End-bulbs. Noticed incidentally by Kölliker, but first investigated and recognised as distinct organs by W. Krause, who named them Endkolben. Their figure in man and apes is usually spheroidal (fig. LXXXVI), but oblong in some quadrupeds. They measure about

2.

Fig. LXXXVI.

3.

th of an inch in diameter, but may exceed this in length with a less breadth, when of an oval shape. They have a simple outer capsule of connective tissue, bearing nuclei, and within this a core of clear soft matter, in which specks resembling fat-granules become visible after exposure to a solution of soda. To an end-bulb there proceeds usually one, but sometimes two, or even three dark-bordered nerve-fibres; and sometimes an originally single fibre divides into two or three immediately before entering the corpuscle; or several branches of one fibre may each run into a sepa rate end-bulb. The fibre or fibres pass into the core, lose their dark borders, and appear to end, when their ends can be traced, in a bulbous extremity or knob. The nerve-fibre, when about to enter the corpuscle, is often much coiled, and this may be the case too with its pale continuation within, which contributes greatly to obscure its actual termination. End-bulbs have been hitherto found in the conjunctiva over the sclerotic coat of the eye, and in the mucous membrane on the floor of the mouth, the lips, soft palate, and tongue, being in these last-mentioned situations lodged in papillæ, or at their roots; also, more deeply, in the skin of the glans of the penis and clitoris.*

1. With two nerve-fibres forming coils within. 2. With one nerve-fibre and fat-granules in the core. 3. Of an oval figure; termination of nerve distinct. Nuclei on. the capsules of 1 and 2.(From Kölliker, after a drawing by Lüdden).

B

Fig. LXXXVII.-END-BULBS IN PAPILLE, MAGNIFIED,
TREATED WITH ACETIC ACID.

A, from the lips; the white loops in one of them are capillaries. B, from the tongue. Two end-bulbs seen in the midst of the simple papillæ. a, a, nerves. -(From Kölliker).

b. Touch-bodies, or tactile corpuscles (corpuscula tactus). Discovered by R. Wagner and

Meissner.

These are mostly of an oval shape, nearly of an inch long, and

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W. Krause has lately described peculiar organs in the skin of the penis and clitoris, allied to the end-bulbs, which he proposes to call genital nerve-corpuscles. They are various in form, but present a mulberry-like surface. One, or two, rarely three or four, dark-bordered nerve-fibres enter each of them. They have a delicate sheath of connective tissue, with many nuclei, and soft finely granular contents allied to the core of the end-bulbs.

of an inch thick. Within is a core of soft, transparent, homogeneous substance, with sparsely imbedded granules; outside, a capsule of connective tissue, with oblong nuclei directed transversely to the axis (and rendered more conspicuous by acetic acid or coloration with carmine), which, together perhaps with some horizontally wound fibres, give the corpuscle somewhat the appearance of a miniature fir-cone. One, two, or even more nerve-fibres, run to the corpuscle, and proceeding straight, or with serpentine windings, approach the summit, up to this point retaining their dark borders; they then pass into the core, and, so far as can be seen, end as fine pale fibres. The touch-corpuscles are found in the skin of the hand and foot, and one or two other parts, where they are inclosed in certain of the cutaneous papillæ, which usually include no vessels. It may be here observed that loops of nerves are sometimes seen in papillæ without touch-bodies, but probably they belong to a nerve on its way to end in the corpuscle of a neighbouring papilla.

Fig. LXXXVIII.-PAPILLE FROM THE SKIN OF THE HAND, FREED FROM THE CUTICLE AND EXHIBITING THE TACTILE CORPUSCLES. MAGNIFIED 350 DIAMETERS. A. Simple papilla with four nerve-fibres. a, Tactile corpuscle; b, nerves. B. Papilla treated with acetic acid; a, cortical layer with cells and fine elastic filaments; b, tactile corpuscle with transverse nuclei; c, entering nerve with neurilemma or perineurium; d, nerve fibres winding round the corpuscle. c. Papilla viewed from above so as to appear as a cross section. a, cortical layer; b, nerve-fibre; c, sheath of the tactile corpuscle containing nuclei; d, core (after Kölliker).

c. Pacinian bodies. In dissecting the nerves of the hand and foot, certain small oval bodies like little seeds, are found attached to their branches as they pass through the subcutaneous fat on their way to the skin; and it has been ascertained that each of these bodies receives a nervous fibre which terminates within it. The objects referred to were more than a century ago described and figured by Vater, as attached to the digital nerves, but he did not examine into their structure, and his account of them seems not to have attracted much notice. Within the last few years, their existence has been again pointed out by Cruveilhier and other French anatomists, as well as by Professor Pacini of Pisa, who appears to be the first writer that has given an account of the internal structure of these curious bodies, and clearly demonstrated their essential connection with the nervous fibres. The researches of Pacini have been followed up by Henle and Kölliker,+ who named the corpuscles after the Italian savant; and to their memoir, as well as to the article "Pacinian Bodies," by Mr. Bowman, in the "Cyclopædia of Anatomy," and to more recent

Abr. Vater, Diss. de Consensu Partium Corp. hum.; Vitemb. 1741, (recus. in Halleri Disp. Anat. Select. tom. ii.) Ejusd. Museum Anatomicum; Helmst. 1750. Ueber die Pacinischen Körperchen; Zurich, 1844.

papers by W. Krause* and Engelmann,† the reader is referred for details that cannot be conveniently introduced here.

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20

Fig. LXXXIX.

The little bodies in question (fig. LXXXIX) are, as already said, attached in great numbers to the branches of the nerves of the hand and foot, and here and there one or two are found on other cutaneous nerves. They have been discovered also within the abdomen on the nerves of the solar plexus, and they are nowhere more distinctly seen or more conveniently obtained for examination, than in the mesentery and omentum of the cat, between the layers of which they exist abundantly. They have been found on the pudic nerves on the glans penis and bulb of the urethra, on the intercostal nerves, sacral plexus, cutaneous nerves of the upper arm and neck, and on the infraorbital nerve. Lately they have been recognised on the periosteal nerves, and, in considerable numbers, on the nerves of the joints. They are found in the foetus, and in individuals of all ages. The figure of these corpuscles is oval, somewhat like that of a grain of wheat,-regularly oval in the cat, but mostly curved or reniform in man, and sometimes a good deal distorted. Their mean size in the adult is from to of an inch long, and from to of an inch broad. They have a whitish, opaline aspect: in the cat's mesentery they are usually more transparent, and then a white line may be distinguished in the centre. A slender stalk or peduncle attaches the corpuscle to the branch of nerve with which it is connected. The peduncle contains a single tubular nerve-fibre ensheathed in filamentous connective tissue, with one or more fine blood-vessels; and it joins the corpuscle at or near one end, and conducts the nerve-fibre into it. The little body itself, examined under the microscope, is found to have a beautiful lamellar structure (fig. xc, ▲). It consists, in fact, of numerous concentric membranous capsules incasing each other like the coats of an onion, with a small quantity of pellucid fluid included between them. Surrounded by these capsules, and occupying a cylindrical cavity in the middle of the corpuscle, is the core, formed of transparent and homogeneous soft substance, in the midst of which the prolongation of the nerve-fibre is contained. The number of capsules is various; from forty to sixty may be counted in large corpuscles. The series immediately following the central or median cavity, and comprehending about half of the entire number, are closer together than the more exterior ones, seeming to form a system by themselves, which gives rise to a white streak often distinguishable by the eye along the middle of the corpuscles when seen on a dark ground. Outside of all, the corpuscle has a coating of ordinary connective tissue. The capsules, at least the more superficial ones, consist each of an internal layer of longitudinal and an external of circular fibres, which resemble the white fibres of areolar and fibrous tissue, with cell-nuclei attached here and there on the inner layer, and a few branched fibres of the yellow or elastic kind running on the outer. The nerve-fibre, conducted along the centre of the stalk, enters the corpuscle, and passes straight into the central cavity, at the further end of which it terminates.

Fig. LXXXIX. — A NERVE OF THE MIDDLE FINGER, WITH PACINIAN BODIES ATTACHED. NATURAL SIZE (after Henle and Kölliker).

The fibrous neurilemma surrounding the nerve-fibre in the peduncle accompanies it also in its passage through the series of capsules, gradually decreasing in thickness as it proceeds, and ceasing altogether when the nerve has reached the central cavity. According to Pacini, with whom Reichert agrees in this particular, the neurilemma form a series of concentric cylindrical layers, which successively become continuous with, or rather expand into the capsules, the innermost, of course, advancing farthest. Others suppose that the capsules are all successively perforated by a conical channel which gives passage to the nerve with its neuriAnat. Untersuchungen; Hanover, 1861, and Zeits. f. rat. Med. xvii. 1865. + Zeits. f. Wiss. Zool. xiii. 1863.