In the Vertebrate (fishes, reptiles, man), the ordinary mode of terrnination of the integumentary nerves is in one or two plexuses, whence the fine terminal branches proceed, and end by dividing into minute branches indistinguishable from the imperfect elastic fibrils of the enderonic tissue. Loops have also been observed, but it is impossible to say whether, in any case, these are real ter minations or not. Gerber and KZ:Uhler have also described " nerve coin" in animals, and in the conjunctiva and lips of man.
The simplest form of sensory appendage in the Vertebrata is presented by the large papillm of fishes, into which a bundle of nerve fibres enters, some of which terminate in the papilla'', while others, whose looped bands may be readily distinguished, probably pass out again.
In certain fresh-water fishes (Barbus, Leu ciscus), Leydig has described papillm of this kind, which have a cup-shaped depression at their extremities, lodging a globular mass of what he describes as modified epithelium.
Special modifications of the tissue of the papillm for sensory purposes in the fingers, tongue, lips, &c. of man have lately been dis covered by Meissner and Wagner, and de scribed by them, under the denomination of the Corpuscula tactics. Milker, who doubts their special relation to the tactile function, on the other hand, prefers to call these bodies, axile corpuscles. They are simply ovoid masses of im - fibrils arranged transversely to the axis of the papilla, so that they appear to be made up of transverse superimposed laminm (fig. 320.). One or two dark-contoured nerve tubules come up through the base of the papilla, and running along one side of the corpuscles, thin out and terminate, without, so far as I have been able to see, entering its substance. In fact, these nerve tubules are, as Kiilliker pointed out, accompanied by a delicate neunlemma, and the axile corpuscle itself appears to me to be nothing more than the enlarged end of this neurilemma.
In Birds, a large proportion of the tegumen tary nerves terminate in bodies which are, on the one hand, related to these axile corpuscles, and on the other to the well-known Pacinian bodies (fig. 322). They are, in fact, usually described under the latter name; but their small size and superficial position, the paucity of their concentric lamellm, and the transverse striation of the solid central axis, ally them closely with the corpuscula tactus. They are found in the skin around the sacs of the feathers, in the beak, and in the interosseus spaces of the fore arm and leg.
A special article (PacrislAsr BontEs) has already been devoted to the organs of this kind which are met with in Mammalia, and it need only be added here, that late re searches have shown that the Pacinian bo dies of mammals, like those of birds, are solid masses of rudimentary connective tissue; the appearance of capsules and of a central cavity, arising merely from the arrangement of the elastic element and the extreme transpa rency of the collagenous substance.* They are
in fact nothing but thickened portions of the neurilemma, and the nerve which they enclose either passes through them, or more usually terminates, more or less abruptly, in the cen tral solid axis.
In the article on the PACINIAN BODIES re ference is made to the peculiar organs de cribed by Savi in the Torpedines. These Savian bodies, in fact, are little more than Pacinian bodies converted into sacs by the development of a cavity between their cen tral and peripheral portions. Now Leydig has discovered that these Savian bodies do not stand alone, but that they form a part of a great series of peculiar integumentary sen sory organs, which are inost characteristically, if not solely, developed in the class of Fishes — the mucous canals and follicles. It has long been noticed, in fact, that in osseous fishes one series of the scales along the sides of the body differ in their structure from the rest, giving rise to what is called the lateral line ; and that a canal runs beneath these scales from the tail to the head on each side ; that then becoming connected with its fellow by a transverse branch over the oc ciput, each canal passes forward on the sides of the head, dividing into two principal branches, one of which following the course of the suborbital bones terminates at the end of the snout, while the other passes down on to the lower jaw. Similar organs, but having a more complicated arrangement, are known to exist in the cartilaginous fishes ; but it is com monly supposed that these canals and follicles secrete the mucus with which the skins of fishes are lubricated. However, in a very beau tiful series of researches, Leydig has shown that the mucus is furnished by the cellular ecderon, and that the so-called mucous canals and follicles are sensory organs. The limits of this article will not permit me to enter into any of the details of structure of these organs, but they may all be described generally as sacs or canals lined by a cellular investment, like that of the skin upon which they open, and filled with a more or less gelatinous sub stance. If the organ be a sac, a single pro tuberant knob, if a canal, a series of them pro ject into the cavity. Each knob is covered by a coat consisting of tiers of much-elongated cylindrical cells. Its substance consists of more or less gelatinous connective tissue, and it receives a nerve (a branch of the fifth or of the vagus), whose fibres divide and become lost in its tissue. In the osseous fishes this nerve usually perforates the peculiarly modified scale of the lateral line, which supports and encloses the canal at these points. In the cartilaginous fishes, the canals have sometimes special fibro cartilaginous coats; or if sacculi, a number of them may be contained in a common cartila ginous investment, as in the Chimmra. Leydig insists with great justice on the identity of the structure of these organs with that of the semicircular canals of the ear.