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HEALTH AND DISEASE 



gata, cerebellum, and brain, giving rise in the first instance to responsive 

 movements of protection or defence, and then successively, as they affect 

 higher arid higher centres, to sensations of sight, hearing, touch, taste, 

 and smell, and finally to ideas, emotions, and intellectual operations. 



The sensory impressions or stimuli thus carried to the nerve centres 

 may not be followed by any visible effect, but in most instances, especially 

 in animals, impulses, which we may conceive to be waves or rapidly- 

 propagated chemical or molecular changes, start, or are liberated from the 

 centres, which travel along similarly constituted cords or nerves, and are 

 conducted to muscles or to glands, exciting the former to contraction 



and the latter to secretion. The cords con- 

 ducting impressions from the organs of sense 

 to the centres are named " afferent" nerves, 

 whilst those which transmit impulses from 

 the centres to the muscles and glands are 

 termed " efferent" nerves. The terms " sen- 

 sory", "motor", and "secretory" nerves are, 

 however, most commonly used. 



The sympathetic system of nerves, 



sometimes named the nervous system of 

 organic life, is destined to regulate the 

 supply of blood to each organ of the body 

 in accordance with its requirements, keeping 

 the blood-vessels contracted when the organ 

 is at rest, but permitting them to dilate 

 under the influence of other nerves when in it 

 is the active discharge of its functions. Thus 

 in the fasting state the stomach is pale and 



quiescent, but in full digestion it is rosy, and performs active movements. 

 The sympathetic system thus, by its action on the vascular system, in- 

 directly but powerfully influences movement and secretion. It is composed 

 of a series of knots or swellings, termed " ganglia ", united to one another 

 by nerve cords. The more important ganglia form a chain lying on either 

 side of the spinal column, and extending through nearly its whole length 

 (fig. 168). Other ganglia belonging to this system, termed "collateral 

 ganglia ", are widely distributed in the body, and give off branches which 

 accompany the blood-vessels, and finally enter the muscular tissue in 

 their walls. The two systems of cerebro- spinal and sympathetic nerves 

 have intimate relations with each other. Their structure is very similar. 



The sympathetic system consists of numerous nerve cords and ganglia 

 distributed over the body, and destined to control and regulate the organs 



Fig. 167. Ganglion Cells of the Sym- 

 pathetic Nerve of the Muscular Coat of the 

 Bladder (magnified about 350 times) 



a, a, , Ganglion Cells. b, b, Their 

 Nuclei, c, c, c, Axis Fibres, d, d, Spiral 

 Fibres. 



