The educational meaning of manual arts and industries . s, usually only at the distal end. A cellmay have several dendrons, but usually only one,sometimes two, and very rarely more than two, neu-rons. Both the dendrons and the neurons are pathwaysfor nervous impulses, it being generally assumed thatthe dendrons convey impulses to the cell body and theneurons carry them away from it. It is not certain,however, that a given branch may not contain twopathways, or^e afferent and the other efiferent. Nerve Development.—According to Professor Don-aldson, a study of a section of the developing spinal
The educational meaning of manual arts and industries . s, usually only at the distal end. A cellmay have several dendrons, but usually only one,sometimes two, and very rarely more than two, neu-rons. Both the dendrons and the neurons are pathwaysfor nervous impulses, it being generally assumed thatthe dendrons convey impulses to the cell body and theneurons carry them away from it. It is not certain,however, that a given branch may not contain twopathways, or^e afferent and the other efiferent. Nerve Development.—According to Professor Don-aldson, a study of a section of the developing spinalcord shows germinal cells dividing and producing 120 MANUAL ARTS AND INDUSTRIES young nerve cells, called neuro-blasts, having a largenucleus and a mass of cytopJasm at one pole, while thiscytoplasm is being gradually drawn into a slenderthread, the beginning of a neuron. As the neuro-blastgrows there are marked changes in the developing cell;first, a great increase in the amount of cytoplasm, andsecond, an enlargement a,nd elongation of the Fig. 4.—Isolated body of a large cell from the ventral horn ofthe spinal cord. Human, X 200 diameters. (Obersteiner.)A, neuron; D, dendrons; N, nucleus with* enclosures; P,pigment spot. In the central system these prolongations extend from the cortex of the cerebrum to the lumbar en PHYSICAL AND PHYSIOLOGICAL RESULTS 121 largements of the cord, and in the peripheral systemthey reach from their point of origin in the cord tothe most distant portions of the extremities. In eachinstance the fiber is a continuous growth of the cellbody. Thus the outgrowths may become quantitativelythe more important portion of the cell, sometimes equal
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