An American text-book of physiology (1897) (14760269526)

Identifier: americantextbook00howe (find matches)
Title: An American text-book of physiology
Year: 1897 (1890s)
Authors: Howell, William H. (William Henry), 1860-1945 Bowditch, H. P. (Henry Pickering), 1840-1911
Subjects: Physiology
Publisher: Philadelphia : Saunders
Contributing Library: Columbia University Libraries
Digitizing Sponsor: Open Knowledge Commons



Text Appearing Before Image:
rth week offetal life.* They divide rapidly and in sucha way that one daughter-cell continues as thegerminal cell, while the other moves awayfrom the primitive surface of the body andbecomes without iurther division a youngnerve-cell or neuroblast. The formation ofneuroblasts in man ceases or becomes veryslow and unimportant by the end of the thirdmonth of fetal life. Two chai-acters of the neuroblast areworthy of careful consideration. First, thereis good indirect evidence that, in early life at least, and before their brancheshave been formed, they are migratory, moving in an amoeboid manner.This being .so, the perfection with which they arrange themselves in the adultsy.stem depends on the accuracy with which they respond to those condi-tions that determine their migration as well as upon the normal character ofthese directing influences (mechanical strain;^ chemotaxis or nutritive attrac- His: Archiv fiir Anatomie und Physiologie, Anat. Abthlg., 1889. His: Unsere Korperfonn, 1874.
Text Appearing After Image:
Via. 145.—PorUon of developing medul-lary tube (human) seen in frontal sectionX 1100 diameters (His): (;, germinal cell;N, neuroblasts. CENTRAL NERVOUS SYSTEM. 611 tiou).^ But with so nuuh lil,erty of movement and with directing influencesthat are so complicated, the chances for deviation from a fixed arrangement are mucli enhanced. Polarization of Neuroblasts.—Moreover, very early in the history of theneuroblast the point on the cell-body from which the neuron will grow ai)pearsin many cases to be fixed, and the cell is thus physiologically polarized.^ Thispolarity being established, the direction in which the neuron first grows isdetermined, and where the cells are misplaced this polarization can lead^tothe confusion of arrangement found in the brains of some congenital idiots.^ The volume of either the germinal cell or of the first form of the neuro-blast was found by His * to be 697 cubic /i in a human fetus (embryo R-length5.5 millimeters, aged 3 to 3.5 weeks). It has jjrevio