. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . dorsal horns of gray matter. The most strikingfeature of this section is the appearance of large numbers of fibres,crossing from the ventral column of each side to the lateral columnof the other. This is the crossing or decussation of the pyramids;here motor fibres from the left hemisphere cross to the right side ofthe cord, and vice versa, so that the left hemisphere of the brain con-trols the right half of the body. This section shows another pe- IN
. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . dorsal horns of gray matter. The most strikingfeature of this section is the appearance of large numbers of fibres,crossing from the ventral column of each side to the lateral columnof the other. This is the crossing or decussation of the pyramids;here motor fibres from the left hemisphere cross to the right side ofthe cord, and vice versa, so that the left hemisphere of the brain con-trols the right half of the body. This section shows another pe- INTERNAL STRUCTURE OP BRAIN-STEM 81 culiarity, as compared with the cord; there is a small mass of graymatter in the midst of the dorsal column. A little higher up the bulb, as the next section shows, the dorsalcolumns are chiefly filled with gray matter, and their white matterhas nearly disappeared; for the fibres which have ascended the cordin the dorsal columns end here, in this gray matter, and their placesare taken by new fibres arising from this gray matter. These newfibres, instead of continuing upward in the dorsal region, promptly. NucJeua V Nuclem X Acceaaory olivarynucleus Olivary nucleusAccessory olivarynucleus Fio. 35.—Section of the Bulb Through the Olive. (Magnified 4 diameters.) (Rauber-Kopsch, Lehrbuch der Anatomie.) cross to the other half of the medulla, and pass upward near themiddle line. The gray matter in the dorsal columns is named thenuclei of the dorsal columns; the crossing of the sensory fibresis the sensory decussation, and the bundle of these fibres whichruns up near the middle line, and which can be traced to the thala^mus, is the fillet, or bulbo-thalamic tract. This is one ofthe chief sensory pathways toward the cerebral cortex. § 16. A section near the upper limit of the bulb shows that thecentral canal has now opened out into the fourth ventricle. Onthe floor of the ventricle are seen the nuclei of the local the nucleus of the ten
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