According to Flechsig, the projection centres differ also anatomically from the association ones, since only the former are connected by centripetal and centrifugal pro jection tracts with the lower brain-centres, while within the association centres such projec tion tracts are altogether wanting. The association areas are connected by fibre-tracts only with the projection centres, from which they receive sensory stimuli ; on the other hand, they may influence the sensory areas by reflex stimuli or inhibition. The associa tion and projection centres also vary in their histological make-up, since the association centres exhibit a specific although uniform texture, while the projection centres present a structure which not only differs from that of the association centres, but varies within the individual fields.
This important theory of Flechsig, however, can no longer be accepted in its entirety. Further investigations have not substantiated the assumption, that only a por tion of the cortex is connected with the lower lying brain-centres by means of projection tracts, since such paths have been proven also for the association fields mapped out by Flechsig. Furthermore, it has been determined, that not only the projection centres possess a special and for each region specific texture, but that there also exists within the asso ciation tract a large number of areas of different structure. As has been shown by the investigations of Vogt and Brodmann, the entire cerebral cortex may be mapped out in numerous fields, which differ from one another in regard to cellular stratification, as well as in regard to fibre-relation, there existing a cyto- and a myelo-architectonic differentia tion of the cerebral cortex (Figs. 114 and 115).
Concerning the relations of each individual anatomically definable field to function, however, we still know very little, and it remains for future physiological and clinico pathological investigations to advance our understanding concerning this problem. With Brodmann we may assume, "that each specific cytological difference must be the expres sion of a definite physiological dignity and that, therefore, all the variously structured cortical fields also preside over different functions. Not, of course, in the sense that one
assigns complex intellectual processes or attributes to specially delimited territories, but in the only warranted sense of Wernicke, who associates only the most elementary functions with definite localities of the cerebral cortex." A question, for which the answer has long been sought, is the existence of definite recollection or memory centres. Many facts point to the actual existence of such memory centres beside the projection centres. Thus, clinical cases are known, in which loss of a perception region was attended with cessation of the corresponding perception, but not of the related memory-pictures ; on the contrary, certain cases with cortical lesions in the immediate vicinity of the perception centres, for example, of the convolutions adjoining the visual and auditory centres, exhibited neither blindness nor deafness, but failure of memory and disturbances of the function of recog nition. Thus lesions of both occipital lobes lead to so-called visual agnosia or perception blindness. The patient may still be able to give information regarding the form and color of an object, but the object itself is unknown, he being no longer able to recog nize the object or, usually, its spatial relations. Further, lesions of the left temporal lobe cause the so-called auditory agnosia or perception deafness, in which condition not only the spoken words, but also auditory stimuli of all kinds are no longer understood. Likewise, lesions situated in the middle third of the postcentral convolution, or farther backward in the parietal lobe, may lead to so-called tactile agnosia, in which, for example, the form of any object no longer is recognized, notwithstanding the integrity of the individual impressions necessary for touch-, space- and muscle-sense.