Galen understood generally the distinction between nerves of sensation and nerves of motion ; but his knowledge upon this point does not appear to have been great, for he supposed that the former proceeded only from the brain, and that the latter had their origin exclusively in the spinal marrow. This opinion is the more remark able, as he himself describes the third pair of cerebral nerves, or principal motor nerve of the eye. In his description of the cerebral nerves he notices the olfactory, though somewhat indistinctly, the optic, the third pair, two branches of the fifth, tho two divisions of the seventh pair, and some branches of the par vagum and hypoglossal nerves; but he appears to have confounded these together very much in his description. He detected the mistake of those anatomists who thought there was an entire crossing of the optic nerves, but fell himself into the error of supposing that no decussation at all takes place.
In order to form correct physiological views, it is necessary to employ many and varied experiments, and to modify them in different ways, that we may be able to 'satisfy the numerous conditions which every problem in physiology presents. To this mode of inquiry Galen sometimes had recourse, and it were to be wished that lie had more frequently made use of it. To prove the dependence of muscular motion upon nervous influence, he divided the nerves which supply the muscles of the shoulder, and found that after the division all power of motion ceased. But he does not seem to have noticed that the nervous influence is only one of the many stimuli which call the muscles into action. As he considered the heart to be devoid of nerves, he might have avoided this error, had he not fortified himself against the truth by assuming that its structure is not muscular. He also deprived animals of their voice by dividing the intercostal mus cles, by tying the recurrent nerve, or by injuring the spinal cord. In theoretical physiology his arrangement of the vital phenomena deserves to be particularly recorded, as it forms the groundwork of all the classifications which have since been proposed. It is founded upon the essential differences observed in the functions themselves. Observiog that some of them cannot be interrupted without the destruction of life, and for the most part are unconsciously performed, whilst another class may be suspended without injury, are accom panied by sensation, and subject to the power of the will, he divided the funetious into three great classes. The vital functions are those whose continuance is essential to life ; the animal are those which are perceived, nod for the most part are subject to the will ; whilst the natural are -performed without consciousness or control. He then assumed certain abstract principles upon which these functions were supposed to depend. He conceived the first to have their seat in the
heart, the second in the brain, and the third in the liver. Thus the pulsations of the hart are produced by the vital forces, and these are communicated to the arteries by the intervention of the pneuma ; this is the more subtle part of the air, which is taken in by respiration, and conveyed from the lungs to the left side of the heart, and thence to the different parts of the body. In the brain the pneuma forms the medium by which impressions from external objects are conveyed to the common sensorium. The same principle is applied to tho explanation of the natural functions also. Observing that these forces are not sufficient for the explanation of the different vital phenomena, Galen had recourse to the doctrine of elements, of which, after the example of Aristotle, and before him Plato in the Timmus,' he admits four, and from the mixture of these deduces the secondary qualities. It may be worth while to observe how he employs this hypothesis in his treatise De tuendS. Valetudioe' (Ed. Johan. Caii, Basil, ap. Froben. 1519), in the explanation of the phenomena of health and disease. The injurious influences to which animal bodies are liable are of two kinds : innate or necessary, and acquired. The former depend upon their original constitution. They are formed of two substances: the blood, which is the material (Me); and the semen, the formative principle. These are composed of the same general elements—" hot, cold, moist, and dry, four champions fierce," or, to express them in their essences instead of their qualities, fire, air, water, and earth. Their differences depend upon the proportions in which these elements enter into their composition. Thus in the semen the fiery and aeriform essences predominate ; in the blood, the watery aud earthy ; and in the blood the hot is superior to the cold, and the moist to dry. The semen again is drier than the blood, but yet upon the whole is of a moist nature ; so that in the original formation of the body there is a predominance of the moist principle. After birth therefore there is a necessity for an increase of the dry principle. This is obtained not from the earth itself, but through the medium of fire. From the increasing influence of this principle, the changes which take place in the body during life are to be explained : as, for instance, the soft ness and flexibility of the limbs in childhood compared with their rigidity in old age. By eating mad drinking we obtain a fresh supply of the dry and moist principles. By respiration and the pulsations of the heart a due supply of the cold and hot principles is kept up ; but as they cannot be obtaiued in a fit state for the different uses of the animal economy, organs are necessary to digest, separate, and remove the unsuitable portions.