The intention of the author in the present paper, is, not to bring forwards any new facts, but to take a general review of the inferences deducible from the series of facts detailed by him in previous papers communicated to this Society. He divides the nerves into two classes, essentially differing in their functions. The first comprehends those nerves, which, proceeding directly from the brain and spinal cord to other parts, convey in the one case to those parts the influence of those organs only from which they originate, and thus excite to contraction the muscles of voluntary motion ; and in the other case transmit to the sensorium impressions made on the parts to which they are distributed. The second class comprises what may betermed the Ganglionic nerves, or those which enter ganglions, properly so called; that term being limited to such protuberances only as receive branches of nerves proceeding from the brain and spiral cord. These nerves are distributed more especially to the vital or gans, as the thoracic and abdominal viscera, and to the muscles subservient to their functions. The nerves belonging to this class also convey impressions to the sensorium, and occasionally excite the muscles of involuntary motion, which, in common with all muscles, possess an inherent power of contractility dependent solely on their own mechanism, and which in ordinary cases are excited by stimuli peculiar to themselves. But the most important function of the ganglionic nerves, is that of supporting the processes of secretion and assimilation, which require for their performance the combined influence of the whole brain and spinal cord. Viewed as a whole, the system of ganglionic nerves, therefore, constitutes, in the strictest sense, a vital organ. Thus the sensorium, though connected by means of the cerebral and spinal nerves only partially with the organs of sense and voluntary motion, is, by means of the ganglionic nerves, connected generally with all the functions of the animal body. Hence affections of the stomach and other vital organs extend their influence over every part of the frame; while those of a muscle of voluntary motion, or even of an organ of sense, although possessing greater sensibility, are confined to the injured part. From a due consideration of the phenomena of the nervous system, it would appear that they imply the operation of more than one principle of action. The sensorial power is wholly distinct from the nervous power; the former residing chiefly in the brain, while the latter belongs equally to the spinal cord and brain, and may be exercised independently of the sensorial power. In like manner, the muscular power resides in the muscles, and may be called into action by various irritations independently of the nervous power, though frequently excited by the action of that power. The muscles of voluntary motion are subjected to the sensorial power through the intervention of the nervous system; and those of involuntary motion are also, under certain circumstances, capable of being excited through the nerves by the sensorial power, particularly when under the influence of the passions. The same observation applies also to other actions which properly belong to the nervous power, such as the evolution of caloric from the blood, and the various processes of secretion and of assimilation. That the nervous power is in these instances merely the agent of other powers, and is independent of the peculiar organization of the nerves, is proved by the same effects being produced by galvanism, transmitted through conductors different from the nerves. The successive subordination of these several powers is shown during death, when the sensorial functions are the first to cease, and the animal no longer feels or wills, but yet the nervous power still continues to exist, as is proved by the nerves being capable, when stimulated, of exciting contractions in the muscles, both of voluntary and of involuntary motion,of producing the evolution of caloric and of renewing the processes of secretion. In like manner the power of contraction, inherent in the muscular fibre, survives the destniction of both the sensorial and nervous powers, having an existence independent of either, although in the entire state of the functions they are subjected to the entire influence of both.
IX . Researches on the intimate structure of the brain.—Second series