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Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER II. SENSATION IN GENERAL. Incoming nerve-currents are the only agents which normally affect the brain. The human nerve-centres are surrounded by many dense wrappings of which the effect is to protect them from the direct action of the forces of the outer world. The hair, -the thick skin of the scalp, the skull, and two membranes at least, one of them a tough one, surround the brain; and this organ moreover, like the spinal cord, is bathed by a serous fluid in which it floats suspended. Under these circumstances the only things that can happen to the brain are: 1) The dullest and feeblest mechanical jars; 2) Changes in the quantity and quality of the blood- supply; and 3) Currents running in through the so-called afferent or centripetal nerves. The mechanical jars are usually ineffective; the effects of the blood-changes are usually transient; the nerve-currents, on the contrary, produce consequences of the most vital sort, both at the moment of their arrival, and later, through the invisible paths of escape which they plough in the substance of the organ and which, as we believe, remain as more or less permanent features of its structure, modifying its action throughout all future time. Each afferent nerve conies from a determinate part of the periphery and is played upon and excited to its inward activity by a particular force of the outer world. Usually it is insensible to other forces: thus the optic nerves are not impressible by air-waves, nor those of the skin by lightwaves. The lingual nerve is not excited by aromatic effluvia, the auditory nerve is unaffected by heat. Each selects from the vibrations of the outer world some one rate to which it responds exclusively. The result is that our sensations form a discontinuous series, broken by enormou...
| ISBN-13 | 9780217746991 |
|---|---|
| ISBN-10 | 0217746993 |
| Weight | 1.06 Pounds |
| Dimensions | 9.00 x 6.00 x 0.73 In |
| List Price | $23.37 |
| Format | Paperback |
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| Pages | 326 pages |
| Publisher | |
| Published On | 2009-08-01 |
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