Other Special Centres.—The cortical centres for vision lie on the median surfaces of the occipital lobes in the neighbourhood of the calcarine fissure. Each half-vision centre—for there is one in each occipital lobe—receives the fibres from the same side of both retinæ. Destruction of one half-vision centre produces the condition known as homonymous hemianopia, in which the medial (nasal) half of one visual field and the lateral (temporal) half of the other is affected, so that there is an inability to see objects situated on the side opposite to the lesion.
Auditory impulses are received in the posterior part of the superior temporal convolution.
Aphasia.—The use of language, spoken or written, as a means of expression depends upon the co-ordination of four different centres: the visual, the auditory, the graphic, and the articulatory. These are situated in different parts of the brain and are connected by sub-cortical association tracts, the main pathway of which lies in the vicinity of the upper end of the fissure of Sylvius. Marie has proved that aphasia results from lesions in this area.
The olfactory and gustatory centres are situated in the uncus close to the pituitary fossa.
Lesions of the frontal cortex anterior to the motor centres, even if extensive, may produce few or no symptoms, and in consequence this region has been called a “silent” area. Occasionally there results a change in temperament or intelligence, and the region is on this account supposed to be concerned with the higher psychical functions. There is evidence that the pre-frontal cortex has a centre for the conscious initiation of movements, and that lesions produce “apraxia,” i.e., inability to perform, or clumsiness in voluntarily performing fine movements such as touching the nose with the finger, though such movements may be perfectly carried out unintentionally. This centre is probably situated in the superior and middle left frontal convolutions in right-handed people. The fibres from the centre to the right motor area cross in the anterior part of the corpus callosum.
Cerebral Localisation.—The various parts of the brain can be localised in relation to the surface by various methods. That devised by Professor Chiene has been found reliable.
Relation of Cerebral Centres to the Surface.—Numerous attempts have been made to formulate rules for locating the different parts of the brain in relation to the surface of the head. The method devised by Chiene is free from many of the difficulties and fallacies common to most other methods, inasmuch as the results obtained do not depend upon making definite measurements in inches, or determining particular angles. Certain fixed and easily recognised bony landmarks—the glabella, the external occipital protuberance, the lateral angular process, and the root of the zygoma—are taken, and connected by lines, which are further subdivided—always being bisected. [Figs. 179] and [181] explain the method. The head being shaved, a line (GO) is drawn along the vertex from the glabella (G) to the external occipital protuberance (O). This line is bisected in M, which constitutes the “mid-point.” The posterior half of the line MO is bisected in T, constituting the “three-quarters point,” and the posterior half TO is bisected in S—“the seven-eighths point.” The lateral angular process (E) is next connected to the root of the zygoma (P) by a line EP, and the root of the zygoma with the seven-eighths point by PS; the line EPS thus forms the base line. The lateral angular process is now joined to the three-quarters point by ET. The two segments of the base line EP and PS are bisected in N and R respectively, and these points connected with the mid-point (M) by lines NM and RM. These lines cut off a part of ET—AB, which is now bisected in C, and from C the line CD is drawn parallel to AM.
Fig. 181.—Chiene's Method of Cerebral Localisation.