A lesion of the _crus_ may in like manner produce spastic hemiplegia and hemianæsthesia of the opposite side, often associated with a lower neurone paralysis of the third and fourth nerves of the same side (crossed paralysis). The optic tract, which crosses the crus, may also be affected, and hemianopia result. Lesions of the _corpora quadrigemina_ cause interference with the reaction of the pupil, disturbance of the functions of the oculo-motor nerve and of mastication, ataxia, and inco-ordination of the movements of the limbs. The symptoms produced by lesions of the _pons and medulla_ vary according to the position of the lesion. If it is unilateral, there may be spastic hemiplegia and hemianæsthesia of the opposite side; if it is situated in the lower part of the pons or in the medulla, there is often also a lower neurone paralysis of one or more of the cranial nerves on the same side as the lesion (crossed paralysis). Paralysis of the external rectus of one eye and of the internal rectus of the other (conjugate paralysis) is frequently found in pontine, and in cortical and internal capsule lesions. _Cerebellar_ lesions are associated with special symptoms. In ataxia, there is inco-ordination of muscular movements, especially of the coarse movements, such as walking. The gait becomes irregular and staggering, with a tendency to fall, sometimes to the side on which the lesion is situated, sometimes to the opposite side. In patients who cannot walk, ataxia may be tested by ordering repeated pronation and supination of the forearm. Paresis or asthenia may be found in the trunk muscles, or evidenced by weakness of the grip, or drooping of the head to one side. Changes in muscle tone may arise and lead to exaggerated or decreased reflexes, often varying from day to day. Vertigo and nystagmus may also be present, in addition to occipital headache and tenderness on percussion. When one lateral lobe is implicated, the symptoms are referred to the same side; when the median lobe is involved, they are bilateral, and there may be retraction of the neck with extension of the legs, probably as the result of the associated internal hydrocephalus.