By others again, these several properties are supposed to be derived from the arteries, which may either supply the materials and construction of that exquisite and peculiar organization, which fits nerves and muscles for performing their respective functions, or may furnish, from the blood, some subtile principle, such as that believed by M. Fontana, to exist there, or such as that we are now examining, which differently modified in different parts, may be the latent cause of all the phenomena exhibited by animals. The advocates for the first opinion observe, that whenever the brain is considerably injured, or its free communication, by means of nerves, with moving parts is interrupted, a deprivation both of sense and motion is the uniform consequence: and, further, that the several organs, both of sense and of motion, appear to suffer detriment from the over strained exertions of the brain in thinking, equal to that which they experience from their own exertions. The second opinion is countenanced by facts and observations not less important. From experiments of Haller; some which are recorded in one of the early volumes of the Philosophical Transactions, and others, it appears that a paralysis of the posterior extremities of animals was induced by tying their aorta. Both Dr Monro and Dr Alexander of Halifax have remarked, that when all the blood vessels, supplying the posterior extremities of frogs, had been divided, and a solution of opium injected under the skin of these extremities, they became, in less than half an hour, both motionless and insensible; whereas, the fore part of the body was not observably affected six hours afterwards; and, in Dr Monro’s experiments, the frogs lived till the day following. Hence Dr Monro concludes, ‘that concomitant arteries, somehow or other, tune the nerves, so as to fit them to convey impression[13].’ On the other hand, where it is intended that nerves shall convey impressions with great accuracy, as in all the senses, and very remarkably in the part which some have amused themselves by considering as a sixth organ of sense, the distribution of blood vessels is more profuse than in almost any other equal part. It is likewise universally true, that increase of vascular action in a part is always attended with a proportional increase of sensibility there. From the valuable experiments made by Mr Cruikshanks, and which have since received the fullest confirmation from those repeated by M. Fontana and others, it appears, that whatever may be the relation between brain and nerves, the latter may certainly be regenerated after excision, and have their functions fully restored. Now, in what manner this can be accomplished, unless by the agency of arteries, would, I imagine, be no easy task to point out.