B. FOCAL LESIONS OF THE ENCEPHALON.—1. FOCAL LESIONS OF THE CEREBRUM, OF ITS CORTEX AND SUBJACENT WHITE SUBSTANCE, AND BASAL GANGLIA.—(_a_) Focal lesions of the frontal lobe produce no specific symptoms, and cannot be directly diagnosticated unless they extend as far caudad as the base of the second and third frontal gyri. The forward mass of the frontal lobe, including the orbital lobule (vide Fig. 10), appears to be inexcitable and insensitive. Even psychic symptoms do not necessarily appear after the loss of considerable amount of cerebral substance from this region. The diagnosis of tumors, abscesses, etc. in this part of the brain must be made by taking into consideration the seat of pain, the presence of cicatrices or other etiological indications, the general signs of the cerebral irritation and compression, but, after all, usually by exclusion. In some cases unilateral anosmia is produced. (_b_) Focal lesions of the caudal part of the frontal lobes, of the insula, the pre- and postcentral gyri, and other parts of the motor zone are usually easy of diagnosis. The symptoms of such lesions have already been detailed when speaking of lesions of the kinesodic system. (_c_) Focal lesions of the parietal, temporal, and occipital lobes of the brain have the characteristic semeiology of lesions of the æsthesodic system, considered supra. (_d_) Lesions of the so-called basal ganglia, the nucleus caudatus, nucleus lentiformis, and thalamus, usually give rise to motor and sensory symptoms indirectly by pressure upon or destruction of the segments of the internal capsule which lie between these bodies (vide Figs. 7 and 9). Lesions of the nuclei caudatus and lentiformis are thus more prone to produce purely motor symptoms, while sensori-motor and ataxic symptoms result from lesion of the thalamus. The symptoms indicating lesions strictly limited (in extent and in effect) to these ganglionic bodies are at present practically unknown. (_e_) Lesions (tumors, etc.) of the lobi optici (tubercula quadrigemina) are productive of early neuro-retinitis and blindness, of convulsions, and of diffused bilateral incomplete paralysis of the body, without symptoms {90} of disease at the base of the brain (vide infra). The diagnosis intra vitam remains of great uncertainty, inasmuch as other lesions in the median line, involving parts adjacent to the lobi optici, may give rise to the same symptom-group. [Illustration: FIG. 10.] [Illustration: FIG. 11.]