The fibres forming the second path pass through the red nucleus in the cerebral peduncle (crus cerebri) and thence by way of the rubro-spinal tract in the lateral column of the cord. The existence of this double motor path explains how after a hemiplegic stroke in which the pyramidal tract is destroyed while the rubro-spinal tract escapes, the patient is able to perform such primitive movements as are involved in walking or standing, while he is unable to carry out finer movements that require higher education. The pyramidal and rubro-spinal tracts, in addition to conveying motor impulses, convey impulses that influence muscle tonus and the deep reflexes. The pyramidal tract conveys impulses that inhibit muscle tonus, while the rubro-spinal tract is the path by which excitatory impulses travel. When the inhibitory influences are cut off, as in a lesion of the internal capsule, the paralysed muscles become spastic, and the deep reflexes are exaggerated. When the excitatory impulses are also lost, as in a total transverse lesion of the cord, the paralysed muscles are flaccid and the deep reflexes disappear. In destructive lesions of the lower neurones, the muscles are always flaccid. The axons passing from the cerebral cortex terminate at different levels in the cord by breaking up into dendrites which arborise around the cells on the grey matter of the posterior horns--this system of cells, axons, and dendritic processes forming an _upper neurone_. From this synapsis the _lower neurone_ proceeds, its axons travelling to the anterior horn and arborising around the motor cells. The axis cylinders pass out in the anterior nerve roots to the spinal nerves and are continued in them to their distribution in voluntary muscles. If the continuity of any group of these lower neurones is interrupted, not only do the nerve fibres degenerate, but the nutrition of the muscles supplied by them is interfered with and they rapidly degenerate and waste, and after an interval show the reaction of degeneration. In addition, the reflex arc is disturbed, and reflexes are lost. As these changes do not occur in lesions of the upper neurones, an appreciation of the differences enables us to distinguish between lesions implicating the upper and the lower neurones. #Sensory Functions and Mechanism.#--Three kinds of sensory impulses pass from the periphery to the brain; (1) deep, or muscular sensibility, (2) protopathic sensibility, and (3) epicritic sensibility. _Deep sensibility_ includes the recognition of (_a_) deep pressure, say by the blunt end of a pencil; (_b_) the position of a joint on passive movement (joint sense); (_c_) active muscular contraction (kinesthetic sense). The fibres that convey these impulses to the spinal cord pass in the afferent nerves from the muscles, tendons, and bones, and so long as these nerves are intact these sensations are retained, even if the surface of the skin is quite anæsthetic.