OPTIC EADIATION AND CHIASMA. 



585 



passes into the optic nerve of the opposite side. The lenticular-nucleus-loop is formed below the 

 lenticular nucleus by the junction of the stria? medullares ; the striae medullares form part of the 

 fibres of the internal capsule, and the inner capsule is largely composed of fibres descending 

 from the cortex. Hamilton also asserts that other cortical connections join the tract as it winds 

 round the pedunculus cerebri, and they include (a) a large mass of fibres coming from the motor 

 areas of the opposite cerebral hemisphere, crossing in the corpus callosum, entering the outer 

 capsule, and joining the tract directly ; (b) fibres uniting it to the temporo-sphenoidal lobe of 

 the same side, especially the first and second temporo-sphenoidal convolutions ; (c) fibres to the 

 gyrus hippocampi of the same side ; (d) a large leash of fibres forming the "optic radiation " 

 of Gratiolet, which connect it directly with the tip of the occipital lobe. There are probably 

 also indirect connections with the occipital region through some of the basal ganglia. Although 

 some observers do not admit the connections w T ith the frontal and sphenoidal lobes, all are 

 agreed as to its connection with the occipital by means of the " optic radiation."] 



[The optic radiation of Gratiolet is a wide strand of fibres expanding and terminating in the 

 occipital lobes. It is composed of, or, stated otherwise, gives branches to (a) the optic tract 

 directly, (b) the corpus geniculatum internum and externum, (c) to the pulvinar and substance 

 of the thalamus, (d) a direct sensitive band (Meynert's " Sensitive band ") to the posterior third 

 of the posterior limb of the inner capsule, (e) fibres which run between the island of Reil and 

 the tip of the occipital lobe (D. J. Hamilton).] 



Chiasma. The extent of the decussation of the optic fibres in the chiasma is 

 subject to variations. As a rule, rather more' than half of the fibres of one tract 

 cross to the optic nerve of the opposite side (fig. 423), so that the left optic tract 



RF 



Fig. 423. Fig. 424. 



Fig. 423. Scheme of the semi-decussation of the optic nerves. L.A., left eye; R.A., right 

 eye. Fig. 424. Diagram of the relation of the field of vision, retina, and optic tracts. 

 RF, LF, right and left fields of vision the asterisk is at the fixing point ; RR, LR, 

 right and left retina the asterisk is at the macula lutea ; l.h., r.h., left half and right half of 

 each retina, receiving rays from the opposite half of the field ; RN, LN, right and left 

 optic nerves ; Ch, chiasma ; RT, LT, right and left optic tracts ; below, the halves of the 

 fields from which impressions pass by each optic tract are superimposed {Goivers). 



sends fibres to the left half of both eyes, while the right tract supplies the right 

 half of both eyes ( 378, IV.). [Thus, the corresponding regions of each retina are 

 brought into relation with one hemisphere. The fibres which cross are from the 

 nasal half of each retina (fig. 424).] 



Hence, in man, destruction of one optic tract (and its central continuation in the occipital 

 lobe of the cerebrum) produces " equilateral or homonymous hemianopia." In the cat there 

 is a semi-decussation ; hence, in this animal extirpation of one eyeball causes atrophy and 

 degeneration of half of the nerve-fibres in both optic tracts {Gudden). Baumgarten and Mohr 

 have observed a similar result in man. A sagittal section of the chiasma in the cat produces 

 partial blindness of both eyes (Nicati). According to Gudden, the fibres which decussate are 

 more numerous than those which do not, although J. Stilling maintains that they are only 

 slightly more numerous. According to J. Stilling, the decussating fibres lie in the centralaxis 

 of the nerve, while those which do not decussate form a layer around the former. 



