800 ANATOMY [NERVOUS believed to be absent from the nerve fibres in the brain and spinal cord, as well as at the peripheral terminations of many nerves. The medullary sheath is a fatty and albu minous substance, which refracts the light strongly. Not unfrequently it collects into little ball-like masses, and sometimes causes irregular bulgings on the fibre, and pro duces a knotted, varicose appearance; at other times it becomes granular, and makes the fibre opaque. By gentle pressure it can be squeezed out of the broken end of a fibre. The axial cylinder is a pale, grey, cylindriform band, usually about one-third or one-fourth the diameter of the fibre, which possesses more tenacity than the medullary sheath, and not unfrequently, as in Fig. 56, 2, projects for some distance beyond the broken end of a fibre. Max Schultze showed that it is not homogeneous, but exhibits a very delicate longitudinal fibrillation, and at the ends of the nerves these primitive fibrillae may separate from each other. Although from its great delicacy the axial cylinder can not be seen in the living fibre of a cerebro-spinal nerve, yet there are many reasons for regarding it as a structure existing in the living nerve, and not the product of a post inortem change. It is the part of a fibre which first appears in the course of development, the medullary sheath and primitive membrane being secondary investing structures, superadded as development proceeds. It forms not un frequently the only constituent of a nerve fibre at its central and peripheral terminations, and is therefore the part of the fibre which is anatomically continuous with the nerve cell, or with the peripheral end-organ. As it is the sole constituent of many nerve fibres at their terminations, and of all nerve fibres in the earlier stage of development, and as it forms the medium of connection between them and the structures in which they terminate, it is obviously of primary importance, both anatomically and physiologi cally, and is believed to be the part of the fibre directly concerned in the conduction of impulses; whilst the investing structures serve the purpose of insulating mate rials. Lister and Turner pointed out, in 1859, that essential differences in chemical composition existed between the axial cylinder and the medullary sheath ; the former being unaffected by chromic acid, though the latter is rendered opaque and brown, and concentrically striated under its influence ; while, on the other hand, the axial cylinder is stained red by an ammoniacal solution of carmine with great facility, although the medullary sheath is unaffected by it. They further showed that these differences in the mode of action of chromic acid and carmine might advantageously be employed in the demonstration of the structure of nerve fibres. Ranke has subsequently stated that the axial cylinder possesses an acid, and the medullary sheath an alkaline reaction. Medullated nerve fibres vary materially in diameter in different parts of the nervous system. In the brain, for instance, they are sometimes as fine as the T^-J-g-jth inch; whilst, in the distributory nerves, fibres of ^-jV^-th of an inch in diameter may be seen ; though it should be stated that, even in the nerves of distribution, fibres of great minuteness are often placed in the same bundle with those of the largest size. Nerve fibres do not branch in their" course, but only at their central or peripheral terminations, and much more frequently at the latter than the former. Non-medullated Nerve Fibres. These fibres, which are characterised by the absence of a medullary sheath, are chiefly found in the sympathetic nervous system, but they occur also in the cerebro-spinal system. The fibres of the olfactory nerve are non-medullated, so also are the peri pheral terminations of the cerebro-spinal nerves, and indeed all nerve fibres in the first stage of their development. In Petromy-on it has been stated that all the nerve fibres are distinguished by the absence of a medullary sheath. Fio. 57. Non- medullated nerve fibres from the sym pathetic sys tem. This form of nerve fibre consists of pale grey, translucent, flattened bands, the T c Virth to ^-uVuth inch in diameter. They usually appear as if homo geneous or faintly granular ; but Schultze showed that, when carefully examined, they present a delicate fibrillated appearance, like that seen in the axial cylinder of a medullated nerve ; so that, like that cylinder, they are supposed to be composed of multitudes of extremely delicate primitive fibrillae imbedded in a finely granulated material. Sometimes these fibres consist solely of this fibrillated material, at other times they are invested by a sheath similar to the primitive membrane of a medullated fibre. Nuclei are also found both in the substance of the fibre and in relation with the primitive membrane. The presence of multitudes of fibres in the sym pathetic nervous system, formed either en tirely, or almost entirely, of a material precisely similar in structure to the axial cylinder of a medullated fibre, and by which the proper function of the fibre can alone, therefore, be exercised, is, of course, an additional argument to those previously advanced, in favour of the existence of the axial cylinder as a normal constituent of the fibre, and of its functional importance. Nerve Cells. Nerve cells constitute an important division of the nervous tissue. They are the characteristic structures in the nerve centres, are susceptible to impressions, or nervous impulses, and are the texture in which the molecular changes occur that produce or disengage the special form of energy named nerve energy, the evolution of which is the distinc tive mark of a nerve centre. The central extremities of the nerve fibres lie in relation to, and are often directly con- tinous with, the nerve cells. It was at one time thought that nerve cells were globular in fonr 1 ; but it is now gene rally understood that, though the body of the cell is not unfrequently globular, two or more processes or poles project from it, and are continuous with its substance. Nerve cells are distinctly nucleated ; the nuclei are usually large, and contain one, and often two nucleoli. The cell substance is granular, and not unfrequently brown or yellow pigment is collected around the nucleus. A cell wall is sometimes apparently present, though at others it cannot be demonstrated. The nerve cells in the grey matter of the brain and spinal cord are imbedded in the neuroglia. In the smaller nerve centres, as the sympathetic ganglia and the ganglia on the posterior roots of the spinal nerves, the nerve cells are surrounded by a capsule of connective tissue. Frantzel, Ivolliker, and others, have described this capsule as lined by an endothelium formed of flattened cells ; and it should be stated that Ranvier has described a similar endo thelium in relation to the connective tissue investment of the cerebro-spinal nerves. It is not improbable that these endothelial cells form the walls of delicate capillary rootlets of the lymphatic vascular system. Nerve cells from which two poles or pro cesses proceed are called bipolar. Charac^ teristic specimens of these cells, as was first pointed out by Robin and R. Wagner, may be recognised without difficulty in the gan glia on the posterior roots of the spinal nerves of fishes, and it is probable that Fia - ss. Bipolar ., ,, i ji T nerre cell, with two similar cells exist in the corresponding nerve fibres con centres in other vertebrates. These cells {hlfspfnlfgangUOT usually possess a globular body, though of a skate,
sometimes it may be elongated; and from opposite points
