THE MESENCEPHALON 



837 



of nerve-cells which give origin to its fibres. It courses caudalward close to the lateral margin 

 of the central grey substance, and is quite small at its beginning in the extreme superior part of 

 the mesencephalon, but as it descends toward the exit of its fibres from the pons, it increases 

 shghtly in size, due to the progressive addition of fibres. Its nucleus also increases slightly 

 in bulk in approaching the region of the chief motor nucleus of the nerve. As mentioned 

 above, the investigations of Johnston and Willems in lower animals suggest that the cells of 

 the mesencephalic nucleus may be sensory instead of motor in character. The sensory 

 nucleus (nucleus of termination) of the trigeminus tapers rapidly and probably does not 

 extend throughout the mesencephalon. 



The nuclei of the trochlear and oculomotor nerves form a practically continuous 

 column of nerve-cells extending close to the mid-line and ventral to the aqueduct 

 of the cerebrum. They are in line with the nuclei of origin of the abducens and 

 hypoglossus, and, like them, may be regarded as an upward continuation of the 

 ventral group of the cells of the ventral horn of the spinal cord. The portion of 

 the column giving origin to the oculomotor nerve is considerably larger than that 

 for the trochlear. 



Fig. 661. — Diagrams showing the Course of Origin of the Trochlear Nerves. (Stilling, 

 The upper figure shows roughly the entire central course of the trochlear nerves; the lower rep- 

 resents their region of exit in transverse section. 



Aquaeductus cerebri 



Nucleus of trochlear nerve 



Decussation of trochlear nerves 



Medial longitudinal 

 fasciculus 



Raphe 



Trochlear nerve 



Trochlear nerve 



■Aquaeductus cerebri 

 Mesencephalic root 

 of masticator 

 Brachium conjunctivum 



•Lateral lemniscus 



A transverse section through the inferior quadrigeminate bodies involves a 

 portion of the decussation of the brachia conjunctiva and the nuclei of origin of 

 the trochlear nerves, while a transverse section through the superior quadri- 

 geminate bodies passes through the red nuclei of the tegmentum and the nuclei 

 of origin of the oculomotor nerves. The latter section will also involve the 

 brachia of the inferior quadrigeminate bodies and the medial geniculate bodies 

 connected with them, and, if slanting slightly forward it will involve the pul- 

 vinars of the thalami and the lateral geniculate bodies. 



The trochlear or fourth nerve is the smallest of the cranial nerves, and is the 

 only one which makes its exit from the dorsal surface of the brain, as well as the 

 only one whose fibres undergo a total decussation. 



Its nucleus of origin is situated beneath the inferior quadrigeminate bodies in the ventral 

 margin of the central grey substance, quite close to the mid-line and to its feUow nucleus of 

 the opposite side, and it is closely associated with the dorso-mesial margin of the medial longi- 

 tudinal fasciculus. Its root-fibres pass lateralward and dorsalward, curving around the margin 

 of the central grey substance, mesial to the mesencephalic root of the masticator nerve. As 

 the root curves toward the mid-line in the dorsal region just beneath the inferior quadrigeminate 

 bodies, it turns sharply and courses inferiorly to approach the surface in the superior portion 

 of the anterior meduUary velum, the frenulum veli. In this it meets and imdergoes a total 

 decussation with the root of its fellow nerve, and then emerges at the medial margin of the supe- 

 rior cerebellar peduncle of the opposite side. Having emerged, it then passes ventrally 

 around the cerebral peduncle, and thence pursues its course to the superior oblique muscle of 

 the eye. It receives optic impulses from the superior quadrigeminate bodies and impulses 

 from the cerebral cortex of chiefly the same side, and it is associated with the nuclei of other 

 cranial nerves by way of the medial longitudinal fasciculus. 



