Diagnosis of Diseases of the Brain and of the Spinal Cord (Paperback)

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: LECTURE II. MEDICAL ANATOMY OF THE BRAIN (Continued): NERVES OF SPECIAL SENSE?OTHER CRANIAL NERVES. Gentlemek,?In our survey of those points in the anatomy of the brain that are of chief medical importance, it may perhaps be most convenient to study next the central relations of the cranial nerves. We will consider first, however, those nerves that subserve the special senses, and will begin with that which is most difficult, of which we have most knowledge, and yet (perhaps therefore) see most clearly how imperfect our knowledge is?the nerve of sight. At the optic chiasma these nerves undergo a partial decussation, rather more than half the fibres crossing. Fifteen years ago this semi-decussation was regarded as satisfactorily proved; but in this questioning age few doctrines seem unassailable, and it has been maintained that there is a total decussation. The result of the discussion that has taken place is to show that the old doctrine is correct, and to place it upon a firmer basis. In some animals there is a total decussation; but this occurs in those creatures in which the eyes are so placed that they never act together, and in which the fields of vision are entirely separate. In proportion as the two eyes are used together, and the fields of vision correspond, the fibres cross, so as to bring the corresponding regions of each retina into relation with one cerebral hemisphere. In man the inner half of each field of vision is smaller than the outer half, because it is limited by the projecting nose. Although the two halves of the retina are of equal size, the sensitiveness of the outer, temporal half (which receives rays from the inner, nasal half of the field of vision) does not extend so far towards the front as does that of the inner, nasal half. This is shown by exam...