THE BRAIN 715 



tract causes blindness in the temporal half of the retina of the same 

 side, and of the nasal half of the retina of the opposite side, resulting 

 in blindness in the field of vision of the opposite side to the lesion. 



A lesion of the optic radiation in the internal capsule results in 

 a corresponding loss of vision a hemianopia of the opposite side. 

 It can be diagnosed from a lesion of the tract by the fact that rays 

 of light thrown upon that part of the retina in which vision is lost, 

 cause a reaction of the pupils to light, since the reflex arc is intact. 

 When the tract is damaged, this reflex is abolished, as the reflex arc 

 for light is broken. Moreover, a lesion of the optic radiation is also 

 associated with the effects due to damage of other tracts of the internal 

 capsule, such as loss of sensation and movement in the opposite half 

 of the body. *<HI 



A lesion of the carebral cortex in the occipital region also induces 

 a hemianopia of the opposite side, in which the light reflex is preserved, 

 but it is not associated with hemiansesthesia or hemiplegia. In this 

 case, the psj'chical processes in connection with vision are interfered 

 with. 



The Caudate and Lenticular Nuclei. The function* of these nuclei 

 are not yet well ascertained. Being parts of the archipallium. or old 

 brain, they are possibly associated with " instinctive " mental 

 processes. 



The Paths in Connection with the Sense oJ Smell. In many of the 

 lower animals the sense of smell is of great importance, and the part 

 of the brain concerned in the sensation of smell is highly developed, 

 and known as the rhinencephalon. While the forebrain owes its 

 evolution to this sense, in higher animals it becomes of less importance. 

 The non-medullated processes of the receptor olfactory cells pass 

 through the cribriform plate of the skull, to end in the olfactory 

 lobe. Each process forms an olfactory glomerulus by interlacing 

 with an arborizing dendrite coming from one of the "mitral" 

 cells of the olfactory lobe. The axons of the mitral cells pass 

 backwards in the olfactory tract. This divides the mesial root, 

 passing inwards to end around the part of the brain known as 

 the callosal gyrus, and also to make connection with the uncus 

 of the opposite side. The exact connections of the lateral root 

 are difficult to follow, but it makes connections eventually with 

 the uncus of the hippocampal gyrus of the cortex (see Fig. 437) and 

 with the thalamus. Fibres concerned in the sense of smell pass in 

 the anterior commissure of the brain, connecting the region of the 

 uncus of the two sides. The fornix is a commissure connecting the 

 hippocampal gyrus and the thalamus. 



