1932] Poliak: Afferent Fiber 8ystem.s, Primate Cerehral Cortex 199 



Chapter XVII 



RESULTS OF THE PRESENT INYESTIGATIONS OF THE 

 VISUAL SYSTEM 



1. VISUAL RADIATION. BOUNDARIES OP THE VISUAL PROJECTION 



CORTEX. CORTICAL TERMINATIONS OF THE 



VISUAL AFFERENT FIBERS 



The uppermost link of the central visual path, the external g-enieulo- 

 cortical or the visual radiation, originates in the external or lateral 

 geniculate body. No evidence was found to show that a portion of 

 the visual radiation might originate either in the lateral nucleus or 

 pulvinar of the thalamus, or in the midbrain (superior colliculus). 

 The visual radiation forms a well definable fiber system, called the 

 external sagittal stratum of the parieto-occipital lobes (H. Sachs). 

 The internal stratum of these lobes is a descending cortico-fugal fiber 

 system terminating in the roof of the midbrain. The tapetum or fiber 

 layer closest to the lateral ventricle is purely a. callosal system. How- 

 ever, other fibers mingle with the external sagittal layer, namely: 

 efferent (passing to the internal sagittal layer), callosal (passing to 

 and from the tapetum), and also association fibers. At oral levels 

 through the parieto-occipital lobes the most dorsal portion of the 

 external sagittal layer is formed by the most caudo-dorsal bundles of 

 the somatic sensory (thalamo-cortical) radiation. 



From its diencephalic origin up to its cortical termination the 

 visual radiation remains strictly unilateral, no portion of it crossing 

 to the opposite hemisphere through the corpus callosum. 



The entire visual radiation is composed of regularly arranged and 

 approximately parallel fiber fascicles. It is a fairly thick fiber sheet 

 or lamina ha-\dng in its totality the shape of a "fan" with its narrow 

 "handle" at the external geniculate body where its fibers lie closely 

 assembled, and its broad wing at its cortical termination. In cross- 

 sections through the occipital lobe its shape is that of a somewhat 

 deformed crescent or sickle which can be divided into three distinct 

 portions: {a) the dorsal, (Z>) the ventral horizontal, and (c) the inter- 

 mediate perpendicular or vertical branch, which connects both 

 horizontal branches. This crescent in its concavity facing medialward 

 embraces (1) the internal sagittal layer, (2) the tapetum, (3) the 



