Among the peculiarities of this portion of the nervous system, one of the most remarkable is, that it is wholly destitute of feeling. Sensibility is inseparably associated with the idea commonly formed of a nerve. But the nervous system consists of two portions, one presiding over sensation and voluntary motion, hence called the sentient and the motive portions; the other destitute of sensation, but presiding over the organic processes, hence called the organic portion. If the communication between the organic organ and the organic nerve be interrupted, the function of the organ, whatever it be, is arrested. Without its organic nerves, the stomach cannot secrete gastric juice; the consequence is, that the aliment is undigested. Without its organic nerves, the liver cannot secrete bile, the consequence is, that the nutritive part of the aliment is incapable of being separated from its excrementitious portion. The organic organ receives from its organic nerve an influence, without which it cannot perform its function; but the nerve belonging to this class neither feels nor communicates feeling, and hence it imparts no consciousness of the operation of any process dependent upon it. Yet there is not one of these processes that does not exert a most important influence over consciousness. How? By a special provision, as curious in its nature as it is important in its result. Branches of sentient nerves are transmitted from the animal to the organic system, and from the organic to the animal; and an intimate communication is established between the two classes. The inspection of fig. XVI. will illustrate the mode in which this communication is effected. A B represents a portion of the spinal cord (one of the central masses of the sentient system), covered with its membranes. The part here represented is a front view of that portion of the spinal cord which belongs to the back, and which is technically called the dorsal portion. [Illustration: Fig. XVI.] 1, 2, 3, 4, 5, 6, 7, 8, 9, the second, &c. ribs with the corresponding dorsal (sentient) nerves, _a_, _b_, _c_, _d_, _e_, _f_, _g_, _h_, going out to supply their respective organs with sensation. C D E, a portion of the main trunk of the organic (non-sentient) nerve, commonly called the Great Sympathetic. F G H, the membrane of the spinal cord cut open and exposing I K, the spinal cord itself, L, the anterior branch of one of the dorsal nerves, arising from the anterior surface of the spinal cord by several bundles of fibres. M, the posterior branch of the same nerve, arising in like manner from the posterior surface of the spinal cord by several branches of fibres. The anterior and posterior branches uniting to form one trunk N. Two branches, P Q, sent off from the spinal (sentient) trunk to unite with the organic (non-sentient) trunk. R S T U V W, other branches of the sentient, connected with the branches of the non-sentient nervous trunks in the same mode.