These contending views do not appear to invalidate the conclusion we are at present justified in drawing, that it is by contact of the nerve elements with the secreting cell, not by their incorporation with it, that the activity of the latter is maintained and controlled. The innervation of the cell of involuntary muscle. In the case of the voluntary muscle the motor end-plates or end-organs are well-defined bodies, and if the termination of the muscle spindle, so well described by Dr. Batten, be viewed with Sherrington as the end-organ of muscular sensibility, these also are no less well defined, and are to all appearance ultimate, and not penultimate, structures. In the case of involuntary muscle, on the other hand, there is still room for controversy as to the nature of the ultimate innervation of the muscle cell. The difficulty of examining this question until recently has been so great, that it is not long since histologists of reputation regarded even so considerable a muscle as the cardiac ventricle as very sparsely supplied with nerves. Even now the "ganglion-free apex " plays a considerable rôle in physiological conceptions of the nature of rhythmical movement. This The Golgi methods have shown us, however, that not only is the cardiac ventricle well supplied with nerves, but that it is so richly supplied that there is reason to believe that ilka muscle cell, to vary the poet's language, has its ain twig o' nerve. is the conclusion come to by Heyman of Ghent,1 who showed many beautiful specimens of the innervation of the frog's heart, prepared by the quick Golgi method, at the meeting of the British Medical Association in London in 1895, and which probably many present had an opportunity of examining. Numerous observations, prior to and since his, lend support to this view. The end-organs described as characteristic of cardiac muscle are knob-like bodies, first observed by Krause, and frequently seen since. What I have taken to be such, however, appear to me to be continuous rather than freely-ending structures, and to be enlargements on the fine fibres of a circumfibrillar plexus, similar to the circumcellular plexuses described by Dogiel in secreting glands. Terminal knobs, it is true, may be observed which do not appear to be continuous with any nervous element beyond them, but they so much resemble small enlargements upon nerve elements, which are undoubtedly continuous with 1 Berl. Phys. Soc., Feb. 17, 1893. similar knob-like enlargements in a continuous chain, that the possibility suggests itself to me, as it did to Dogiel in the case of glandular nerves, that defective staining may account for the apparently abrupt termination of the fibril in some cases. The majority of present-day histologists, however, appear to be as convinced of the free ending of muscular nerves in all involuntary structures as they are of such a termination in the case of voluntary muscle. It is certainly difficult to conceive how otherwise the separate functions of nerve elements, of which there is good physiological evidence, can be secured. In the light of modern histology, the ganglionic plexuses which innervate the muscular fibres of the intestine and bladder must be regarded as penultimate, not ultimate, structures. The plexuses, for example, of Auerbach and Meissner, treated by the Golgi method, reveal a far-reaching system of fine processes. This is well shown by a specimen of Kölliker's, which I saw at Würzburg, and which is figured in his book.1 The drawing is a minutely faithful delineation of the original preparation from the plexus of Meissner of a kitten two days old. The innervation of the intestinal muscles is probably double, as Gaskell and others have pointed out,2 non-medullated fibres of the vagus supplying the circular muscles throughout the greater part of the alimentary canal, and viscero-motor fibres from spinal nerves, the longitudinal fibres. The discrimination by the microscope of motor fibres from more sources than one, and of motor from sensory fibres in involuntary muscles, is no more possible at present than it is to distinguish secretory-motor from sensory fibres in glands. As regards the detection of sensory fibres, in the case of those supplying such special sense organs as the Pacinian bodies of the mesentery, and similar bodies which have been found in other organs (for example, the pancreas), we are justified in assuming that we are dealing with a sensory nerve termination. But no such guide to the detection of a sensory, as distinguished from a motor, nerve-ending is at present known in involuntary muscular fibre. If, however, we agree with Kölliker that a medullated nerve-fibre, found in the distant periphery, is to be regarded as cerebro spinal and sensory,3 we may have in this a guide to the penultimate distribution of sensory nerves in the viscera. 1 "Gewebelehre," Fig. 843, S. 868. 2 Journ. Physiol., Cambridge and London, vol. vii. p. 18. Specimens from the pelvis of the kidney of a mouse (Figs. 1 and 2) show the fine fibres arising from such a sourcee-a plexus, the fine subsidiary divisions of which are countless, but which, in view of the present opinion of the majority of histologists, may be assumed ultimately to end free, as indeed at some points it appears to do in the sections which are figured. In the case of the heart, also, similar plexuses may be observed coursing transversely across the muscle bundles. While these probably terminate parallel to the fibres, giving off twigs to the FIG. 1.-Dark bordered sensory nerve (a) from the pelvis of the kidney of a mouse; (b) nodal point whence a branch (c) springs, highly magnified; (n) nucleus of sheath. cells, they are conceivably from another source than some of those found running parallel to muscle fibres, and terminating or appearing to terminate in the end-bulbs already mentioned. The importance of this double cardiac nerve supply we shall have to deal with later. Such plexuses are shown in Fig. 3. Future research by staining methods, or by these in combination with the Wallerian method of degeneration, may enable us more accurately to distinguish between the ultimate distribution of visceral motor and sensory nerves, to the benefit of correct anatomy, and as affording a valuable check upon ingenious physiological surmise. The The nervous mechanism of excretion and metabolism. essential conditions for excretion are a medium containing the FIG. 2.-Penultimate plexus from the pelvis of the kidney of a mouse. FIG. 3.-Penultimate plexus from the heart of a mouse. (a) Nuclear excretion and vessels to convey it to a point at which it is to be excreted, where appropriate arrangements for its conveyance out of the body exist. If we take the kidney as exhibiting these conditions in a typical manner, we shall be able to study the nervous anatomy of this process, so far as we at present know it. While this organ, however, exhibits the essentials of excretion mentioned, it is also one beset with considerable difficulties for the histologist of the peripheral nervous system. Its close texture and general density render the use of staining methods, as applied to the finer nerve elements, a difficult matter. So great a master in histological FIG. 4.-Innervation of a vessel in the kidney. (a) Ganglionic cluster; methods as Gustaf Retzius recognised and acknowledged this difficulty, and stated that even by Golgi's method the nerves of the kidney did not stain well.1 He succeeded, however, in staining the vascular nerves as far as the glomerulus, but not further, as many specimens show. My own attempts to use the chrome-silver method for this purpose were made before my visit to Würzburg, and resulted in little beyond the staining of blood vessels and a few questionable nerves. A specimen, however, which Herr Hofmann gave me, shows the vascular innervation of the kidney very well. Hofmann also found the organ difficult to stain by the Golgi method, and did 1 Biol. Untersuch., Stockholm, N. F., Bd. v. S. 35. |