In enquiring into the function of a nerve, the first point to determine is its anatomy, where by we learn whether it be distributed to mus cular parts or to sentient surfaces ; and then to ascertain whether its distribution in man corre sponds with that in the inferior animals. Ana 1 tomy, human and comparative, affords by far the most certain grounds to enable us to decide upon the endowment of a nerve : if the nerve be distributed to muscular parts, it is evident that it cannot be a merely sentient nerve, although it may contain some sentient fibres.
Experiment upon animals recently dead also affords considerable aid in reference to questions of this k ind. Mechanical or chemical or gal van ic irritation of a nerve will cause muscular contrac tions if it be a motor nerve, and will produce DO perceptible change in either nerve or muscles if it be not muscular. Under certain circum stances, however, simple irritation of a nerve, while it evinces no change in the nerve itself or in the parts with which it is connected, will affect the portion of the nervous centre in which it is implanted, and will through that excite certain motor nerves to stimulate their muscles. To affect motor nerves through sensitive ones, it is generally necessary to stimulate their peripheral fibres, the entire trunk remaining uninterrupted in its course; and it would appear as if a cer tain peripheral organisation, as for instance a developement of papillw on the tegumentary surface, were necessary for this purpose. Very rarely irritation of the trunk of a sentient nerve produces this effect; the least equivocal in stance indeed in which, so far as I know, inuscular action can be produced in this way, . e., by irritation of the central segment of the trunk of a'nerve, is in the case of the glosso )haryngeal nerve. Dr. John Reid has suc :eeded, after section of this nerve, in producing :ontraction of the pharyng,eal muscles by stir-I:m ating its central segment.
MM. Longet and Matteucci affirm that a motor nerve may be distinguished from a com pound one by the different effect produced on each by opening or closing a galvanic current, according to the direction in which it passes in the nerve. We have referred above to the results of experiments on compound nerves, the sciatic for instance, by means of the electric current. Compound nerves, as has been shown by these means, may at first be affected equally on opening as on closing the circuit, whether the current be direct or inverse ; but after a time they are excitable, as shown by the con traction of the muscles below the point stimu lated, only on closing the direct current or opening the inverse. With a purely motor nerve, however, such as the anterior root of a spinal nerve, a different result is obtained after the first period has passed ; inasmuch as the contractions of the muscles can only be excited on opening the direct current or closing the in verse.*
Experiment upon living animals likewise affords us some assistance in determining the functions of nerves. This mode of inquiry, how ever, must be used with great circumspection, and great caution must be observed in the inter pretation of the results which it elicits. Section of a nerve paralyses its function, and occasions loss of motor or of sensitive power, according to the nature of the parts to which the nerve is distributed. Experiment of this kind, however, frequently leads to very unsatisfactory results, because it is often a matter of extreme difficulty to reach the nerve in question; the operation for that purpose may involve other parts and nerves as well, and sometimes it may be impos sible to divide one nerve without injuring ano ther immediately adjacent to it. Moreover, the shock of a severe operation frequently produces so much disturbance in the entire system of the animal as to render it extremely difficult to form any accurate opinion as to the effects of the section of the nerve under examination.
Clinical medicine gives very important aid to physiological enquiries of this nature. Disease or injury of certain nerves impairs or destroys or modifies certain functions. The various forms of partial paralysis, especially those affecting the face, may be referred to in illustration of this assertion. Thus a very dis tinct series of signs accompany disease of the facial nerve or the portio dura of the seventh pair; and these signs mark it very distinctly as the motor nerve to the muscles of the features, and to the orbicular muscles of the eyelids. Clinical research, indeed, taken in conjunction with anatomy, forms the basis of our present accurate knowledge of the office of this nerve. In like manner we learn that loss of sensibility of the face is dependent on disease affecting the fifth nerve, and from the parts of the face which are affected by anwsthesia we can tell what portions of that great nerve are diseased. lIere again anatomy and clinical medicine have mainly contributed to the advance of our know ledge. The partial palsies which affect the muscles of the eye ball likewise give very dis tinct interpretation to the functions of these nerves, such as the third and sixth, the action of whose muscles is well understood. Many other instances might be quoted which clearly show that, while clinical medicine and anatomy are of infinite service in building up and con firming our knowledge of the function of nerves, this knowledge, in its turn, does great service in increasing the facility with which we can distinguish disease.