. Physiologie humaine : conçu pour les collèges et les classes supérieures dans les écoles, et pour la lecture générale. Hichmake up son tronc. 235. Les cellules qui forment la particularité de la structure de la substance grise sont souvent, comme vous l'avez vu dans la Fig. 69, d'apparence verysingulière de leurs prolongations. Ils se trouvent dans les thainterstices d'un réseau vasculaire. Un approvisionnement en sang artériel est absolument essentiel à la performance vigoureuse des fonctions de la substance grise. Si l'alimentation est coupée d'une manière quelconque, comme par l'échec de l'action des coeurs dans l'évanouissement, l'insensibilité et la perte de la puissance du mouvement

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. Human physiology : designed for colleges and the higher classes in schools, and for general reading. hichmake up its trunk. 235. The cells which form the peculiarity of the structure ofthe gray substance are often, as you saw in Fig. 69, of verysingular appearance from their prolongations. They lie in thainterstices of a vascular network. A due supply of arterialblood is absolutely essential to the vigorous performance of thefunctions of the gray substance. If the supply be cut off inany way, as by the failure of the hearts action in fainting, insensibility and the loss of the power of motion are the conse-quence. While the gray substance is on the outside of thebrain, it is on the inside of the spinal marrow. It is also onthe inside of the little bodies called ganglions, scattered hereand there, as depositories of nervous force, or as little brains, aswe may term them. These ganglions are not merely a part ofthe apparatus of communication. They are different fromplexuses, which are mere combinations of nervous trunks, asseen in Fig. 77, t t being the trunks, which, after uniting with. each other in various ways, again separate to go to theirdifferent destinations. At g, in Fig. 76, is a ganglion intowhich the fibres f of the nerve n run. It then divides agaiuinto branches b. These ganglions produce nervous force, andtherefore are composed like the brain in part of gray substance.The spinal marrow, too, produces as well as transmits, and so 13* 150 HUMAN PHYSIOLOGY. Changes in the nerve-cells. Termination of the nervous fibres. this substance forms a part of it. This gray substance, as it isin constant operation, is subject to much wear and tear, as womay express it, and therefore the changes of repair are con-stantly going on in its structure. Hence, the necessity for solarge a supply of blood, as is secured by the network of vessels, among which the cells peculiar to this substance are scattered.The microscope has fully demonstrated the reality of thesecontinual c

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