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from a nervous packet, in the manner before mentioned, though too minute to admit of further subdivision, appear by the microscope to be themselves packets of smaller threads. It is generally asserted by microscopical observers, that the i nerves and medullary matter of the brain and spinal marrow are the same, and are composed of very minute fibres. Fontana speaks confidently on this pointi; and he further says, that he has seen these nervous : fibres regenerated in the medium which has been formed to unite a divid-> ed nerve.'10 He describes the nervous fibrės in every part of the nervous system as cylindrical, pursuing a slightly undulating course, and being in a consis derable degree transparent. He states also that they are larger than the ultimate fibres of muscles.
Microscopical observers also tell us, that
though the nervous fibrils in each packet appear distinct, and may be separated from each other in the manner already described, yet they have nevertheless, transversé communications with each other, 1 Eachi nervous fibre has been supposed to be covered by investing membranes similar to those of the brain; but this opinion is founded on an analogy with what is observed in the optic nerve, rather than on act tual observation with respect': to others. That they have investing membranes is clear, and we are told that we may dissolve the medullary or netvous matter by an alkali, and leave these investing membranes; or on the other hand, that we may dissolve the investing membranes by nitric acid, and leave the medullary fibres.
! I OSKI Having thus spoken of the chief cireumstances relating to the anatomy of
the nervous system, I shall not dwell on this part of the subject, but hasten to the principal object of the lecture, to consider its Physiology, in order to examine how far Mr. Hunter's Theory of Life, seems adequate to explain the phænomena of the nervous functions.
First then, it is generally believed that all sensation is in the brain, and that all volition proceeds from that organ. This proposition requiring to be impressed so as to produce conviction, for it is the foundation on which all our future reasoning is founded, I shall state the principal causes of this opinion. First, If the continuity of a nerve be intercepted at any point between that extremity which receives impressions from the objects of sense, and which therefore may '
be called the impressible or tangible extremity, and that which communicates with the brain,
and is usually called its sensorial extremity, both feeling and volition by means of that nerve are suspended.
2dly. If a certain degree of pressure be made upon the brain, both feeling and voluntary motion cease whilst it continues and return when it is removed.
3dly. As we have evidence that the perceptions and intellect of animals increase in proportion as the brain becomes larger and more complex, so we have reason to conclude that these faculties are connected with that part of the nervous system.
4thly. The conviction which we generally though not constantly experience, that feeling exists in the part which receives impressions, is shewn to be deceptive by the
following facts. If a nerve be irritated midway between the brain and its extremities; severe pain is supposed to be felt in those extremities; and if it supplies muscles, those muscles become convulsed. Thus when a disease forms about the hip joint, or in the loins, many persons have applied poultices to their knees, from a conviction that as the pain was felt in the knee, it was the seat of the disorder. In like manner, persons who have had their limbs amputated, can scarcely believe that they are removed, because of the pain and other sensations they still seem to feel in them. In either of these cases, motions being excited in the middle of nerves, and transmitted to the brain, are attributed to a disordered state of those parts from which such motions have heretofore originated.
If then it be admitted that sensation