Quain's elements of anatomy . that a few of the fibres do not thus decussate. The diffi- 364 ORIGIN OF THE CRANIAL NERVES. culty of tracing the course of the fibres in the chiasma is very great in conse-quence of the fact that they take, not a straight, but a curved course withinthe commissure. Tlie fibres of the optic tract come from the anterior quadri-geminal body through its brachium ; from the inner geniculate body; from theexternal geniculate body and the thalamus ; and lastly a few are said to passinto the anterior part of the tract, where this is in contact with the tubercinereum, from
Quain's elements of anatomy . that a few of the fibres do not thus decussate. The diffi- 364 ORIGIN OF THE CRANIAL NERVES. culty of tracing the course of the fibres in the chiasma is very great in conse-quence of the fact that they take, not a straight, but a curved course withinthe commissure. Tlie fibres of the optic tract come from the anterior quadri-geminal body through its brachium ; from the inner geniculate body; from theexternal geniculate body and the thalamus ; and lastly a few are said to passinto the anterior part of the tract, where this is in contact with the tubercinereum, from the basal ojDtic ganglion of Meynert (see p. 327). According toStilling a bundle of fibres passes to the optic tract from the corpus subthalami-cum. and another bundle can be traced up to it from the pyramidal decussationin the medulla (Arch. f. micr. Anat. xviii, 1880). The third or oculomotor nerve (fig. 313, iii) arises from a column oflarge yellowish cells, ou either side of the middle line, in the grey matter Fig. Fig. 313.—Base of the brain with the origins of the cerebral nerves (Allen Thomson). J This figure is taker from an adult male brain which Lad been hardened in alcohol. 1, superior longitudinal fissure ; 2, the olfactory tract and sulcus ; 2, orbitalconvolutions ; 2, inferior frontal convokition ; 3, 3, 3, fissure of Sylvius ; 4, 4, 4,temporo-sphenoidal lobe ; 5, 5, occipital lobe ; 6, on the right anteiior pyramid of themedulla oblongata above the ? decussation ; 7, amygdaloid lobe of the cerebellum ;8, biventral lobe; 9, slender lobe; 10, posterior inferior lobe ; +, the inferior vermi-form process ; I, olfactory bulb ; I, the tract divided on the left side ; II, in theanterior perforated spot, njarks the Iight optic nerve ; the left has been cut short ; III,on the right crus cerebri, denotes the third nerve ; IV, the fourth nerve ; V, the fifth ;VI, on the pons Varolii, the sixth ; VII, also on the pons Varolii, the facial with theauditory nerve on it
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