_Protopathic sensibility_ is of a lower order than epicritic. It consists in the recognition of painful cutaneous stimuli and of extreme degrees of heat and cold. The fibres concerned are non-medullated and regenerate comparatively quickly after injury, so that protopathic sensibility is regained before epicritic. _Epicritic sensibility_ is the most highly specialised and permits of the recognition of light touch, _e.g._, with a wisp of cotton wool, of fine differences of temperature, and of discriminating as separate the points of a pair of compasses 2 cm. apart. These sensations are carried by medullated nerve fibres, and are slow to return after injury to the nerves. The sensory nerve fibres conveying these different impulses pass to the ganglionic cells of the posterior nerve roots. From each of these cells a process passes into the cord and bifurcates into an ascending and a descending branch. In the cord the fibres rearrange themselves and pass to the brain by a double path. Those that convey sensations of pain and of temperature pass by the spino-thalamic route by way of the tract of Gowers and the fillet to the optic thalamus; those that are concerned with the muscular sense, the joint sense, and tactile discrimination pass up the posterior columns in the tracts of Goll and Burdach to the nuclei gracilis and cuneatus in the medulla, whence they pass to the optic thalamus. From the cell station in the optic thalamus the fibres proceed to the _cortical sensory centres_, that for tactile sensation being situated in the post-central (ascending parietal) gyrus; that for muscular and stereognostic sense lying probably in the adjacent portions of the parietal lobe. In a unilateral lesion of the cord, pain and the temperature sense may be disturbed in one limb, and motor power and tactile sensibility in the other, as the fibres that convey impressions of pain, and those that subserve the discrimination of temperature, pass up and decussate in the cord a few segments above their point of entrance. [Illustration: FIG. 180.--Diagram of the Course of Motor and Sensory Nerve Fibres.] #Effects of Lesions of the Motor and Sensory Mechanisms.#--Lesions of the _motor mechanism_ differ in their fundamental characters according as they affect the upper or the lower neurones. The signs also vary according as the affected area is _destroyed_ or merely _irritated_, say by the pressure of a tumour. Irritative lesions in general produce muscular spasms or convulsions, while destructive lesions cause paralysis. The essential differences in the effects of destructive lesions of upper and lower neurones may be indicated thus:-- _Upper Neurone Lesion._ _Lower Neurone Lesion._