. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . Fia. 21.—Brain of an Embryo of About Three Months.(His.) Inter-brain and mid-brain are already concealedby the pallium; the cerebellum and bulb are shown; also,through the fissure of Sylvius, some of the olfactory re-gion can be seen. 54 THE NERVOUS SYSTEM IN THE INDIVIDUAL § 11. The cortex develops late in comparison with other parts ofthe nervous system. Nerve-cells begin to appear here at about thebeginning of the third month of foetal life, or mor
. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . Fia. 21.—Brain of an Embryo of About Three Months.(His.) Inter-brain and mid-brain are already concealedby the pallium; the cerebellum and bulb are shown; also,through the fissure of Sylvius, some of the olfactory re-gion can be seen. 54 THE NERVOUS SYSTEM IN THE INDIVIDUAL § 11. The cortex develops late in comparison with other parts ofthe nervous system. Nerve-cells begin to appear here at about thebeginning of the third month of foetal life, or more than a month laterthan in the cord (compare Fig. 20). The appearance of the axonsand dendrites of the cortical cells is also late, and indeed the cor-tex is by no means fully developed at birth. The specialization ofthe pallial wall into a nervous structure, with gray cortex and under-lying white matter, begins at a definite part of the wall; namely,where it joins the corpus striatum, or at the junction of the pallial. Fig. 22.—Frontal Section of the Brain at the End of the FourthMonth. (His.) The callosum appears clearly. At each side ofthe figure a depression in the wall of the hemisphere indicatesthe fissure of Sylvius. outgrowth with the main stem of the neural tube. From here, thedevelopment of a cortex spreads up over the lateral surface, andfinally over the top down into the median fissure between the hemi-spheres. Near the bottom of this fissure the cortex ends, leaving anarrow strip of the wall free from the usual covering of gray exposed strip of white matter of the right hemisphere is in closeproximity to that of the left, and at a later stage the two strips growtogether in the central portion of their extent, and thus the white mat-ter of the two hemispheres becomes continuous. Axons, in vastnumbers, here cross over from one hemisphere to the other, bring-ing the two into functional connection. This bridge between thehemispheres is the corpus callo
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