Termination tf nerves.—Under this term we describe the peripheral connexion of nerves with the various tissues and organs, and it is much to be regretted that our knowledge in reference to that connexion is scarcely more complete or accurate than that of their origin. The only instance in which we can speak pretty confidently respecting the peripheral connection of nerves, is with regard to mus cles. In the striped muscle, nerves appear to form loops, the convexities of which are di rected across the fibres of the muscles. Each nerve-fibre passes at first parallel to the direc tion of the muscular fibres, and then crosses them in an arched form to pass back into the bundle from which it had emerged, or to be mingled with the fibres of some neighbouring bundle, passing back in it to the centre, pro bably to some part of it different from the place of origin of the nerve. As far as present means of observation enable us to judge, there does not appear to be any other connexion be tween the nerve-tubes and the muscular fibres beyond the simple contact of the tubular mem brane of the former with the sarcolemma of the latter. We have no evidence of any mingling of the true nerve-substance with the sarcous elements, and, therefore, we are forced to con clude that whatever be the nature of the in fluence which nerve exerts upon muscle to pro voke it to contraction, that influence is exer cised through the two layers of homogeneous membrane which form the investments of the nervous and sarcous elements respectively.
The best mode of observing the disposition of nerve in muscle is to examine under the microscope very thin and transparent muscles of some small animals. The abdominal mus cles of the frog first afforded toles, and long after him to Prevost and Dumas, this opportu nity; the muscles of the eyeball in small birds were used by Valentin; Burdach examined the muscles of the frog's tongue; have found the intercostal muscles of the mouse very suitable for the purpose.
Peripheral expansion of nerves on sentient surfaces.—With reganl to the disposition of nerves on sentient surfaces (the skin, for ex ample) the most probable view appears to be that they are disposed in a plexiform manner. The nervous trunks pass toward the surface dividing and subdividing, the ramifications pass ing backwards to the centre in conjunction with neighbouring bundles; so that, whilst a very intricate plexus is formed, the looped arrange ment, similar to that described in muscle, pre vails, the convexities of the loops being di rected towards the deep surface of the integu ments. Gerher states, that in those parts of the skin which are provided with papillx, the nerve-loops pass into the bases of the papilhe and form an element of their composition; and he adds, that in some instances the nerve-tube which forms the loop exhibits tortuosities or convolutions similar to those which are seen upon bloodvessels. According to the same author, in parts of the skin where the tactile sensibility is acute, the meshes uf the nervous plexus are extremely small, whilst they are of large size where the skin is not highly sensitive.
The arrangement of the primitive fibres in loops has been seen by lienle on some parts of mucous membrane, in the membrana nicti tans of the frog for example, and in the mu cous membrane of the throat in the same ani mal. A similar disposition has been described
and delineated by Valentin on the pulps of the teeth. (Fig. 333.) Retina and optic nerve.—The examinatio of the peripheral connexions of the nerve of pure sense has not thrown light on the general question. The peripheral expansion of the optic nerve or the retina presents all the elements of a nervous centre; the grey matter is present in it in considerable quan tity, and certain fibres continuous with the pri mitive nerve-tubes are likewise expanded in It. But the connexion of these fibres with the grey matter has not been datected here any more than in the centres themselves, nor has any arrange ment of looping or of plexuses been demon strated. Alr. Bowman has been led, by recent examinations, to the opinion that these fibrei are the central parts of the nerve-fibre (the bands of Remak) which have been deprived of the tubular membrane and of the white substance of Schwann. It is worthy of notice that the so called optic nerve itself presents certain peculiar characters, which entitle it more to be regarded as a prolong,ation of the nervous centre rather than as a distinct nerve. The nerve-tubes which are met with in it are for the most part of very minute size ; they admit of separation with great difficulty, owing to their not being dis posed in fascicles like those of other cerebro spinal nerves : they appear to be surrounded by and deposited in an abundant granular blastema, in which there seems to be some scattered elements of grey matter. These characters, with the peculiar construction of the peripheral expansion, would induce me to regard what is generally described as the optic nerve as a process of the brain itself, around the peripheral portion of which a dioptric appa ratus has been disposed in order to produce those refractions in the rays of light which are necessary to the formation of an image upon the retina. And my friend, Mr.Bowman, has been led to adopt a similar conclusion from examin ing the structure of the retina and optic nerve.
ONctory nerves.—The true olfactory nerves are very numerous and pass from the bulb of the olfactory process or olfactory nerve of descrip tive anatomists. The peculiar characters of this process, as distinguishing it from a nerve properly so called, have long attracted attention. In truth, this process has the characters of a portion of the brain in a much more obvious way than the optic nerve, for it contains a larger portion of grey matter which adheres as a distinct layer to the white matter, as in the formation of the convolutions ; and, moreover, its anterior ex tremity or bulb contains a ventricle which may easily be demonstrated in a recent brain. It is from this bulb that the minute threads, which may be properly called olfactory nerves, take their rise and pass down through the foramina of the cribriform plate. Nothing satisfactory is known as to the disposition of the ultimate ramification of these nerves upon the Schnei derian mucous membrane. The statement of Valentin that they form loops similar to those of cutaneous nerves is probably correct. It is not improbable that the papillw described by Treviranus were particles of columnar epithe lium to which cilia are attached.