80 THE NERVOUS SYSTEM 



i 



reflexes. It extends from the top of the medulla oblongata (the medulla 

 is usually considered a part of the cord), downward in the canal of the 

 spinal column, to a point opposite the body of the second lumbar vertebra; 

 thence its large sacral roots continue as the cauda equina. The cord, 

 as a whole, is cylindrical, but there are two prominent enlargements 

 laterally, one in the cervical and the other in the lumbar region. The 

 former swelling is due to the presence of the nerves of the arms, the 

 latter to those of the legs. The cord is also made up of gray matter and 

 white matter, the former within the latter, but passing outward through 

 it as the anterior and the posterior roots. The gray matter, as seen on 

 cross-section of the cord, is shaped much like the two extended wings of 

 a moth connected by a narrow commissure. In the center of the latter is 

 the canal of the cord, a minute tube (sometimes called the sixth ven- 

 tricle), lined with ciliated epithelium (beating upward) and rilled with 

 the cerebrospinal lymph. The posterior end of each of these gray- 

 matter wings is relatively slender and long, and is continuous with the 

 posterior (afferent) spinal roots extending outward through the white 

 matter. The anterior end of each wing is rounded and short and is sur- 

 rounded by white matter except where the anterior (efferent) spinal roots 

 pass outward. Thus these two pairs of roots divide each half of the cord 

 into three unequal columns called anterior, lateral, and posterior. The 

 whole cord is nearly divided dorsoventrally by two fissures, the dorsal 

 being deeper but narrower than the ventral or anterior fissure, and with- 

 out a commissure of white matter intervening between its inner end and 

 the gray commissure. 



CONDUCTION. Let us look in turn at the three general functions of 

 the cord : conduction, distribution and collection, and reflexion. The 

 cord is the great highway between the legs and the lower part of the 

 trunk and the upper parts of the central nervous system. Numberless 

 impulses passing upward and downward between these regions go 

 directly in the white matter, and sometimes without alteration or loss, 

 probably, the whole length of the cord. In general terms those impulses 

 passing upward (called afferent, centripetal, or "sensory") go in the 

 posterior or dorsal parts of the cord, while those that go downward 

 (efferent, centrifugal, or "motor") pass in some part or other of the 

 anterior or ventral regions. Owing to this and other far-extended 

 divisions of labor, not only are there present the three gross columns 

 already noted, made accidentally as it were by the roots passing through 

 the white matter, but there are present in each of these numerous smaller 

 bundles of fibers. Only a few of these, doubtless, have been located 

 without question and named. The methods by which these tracts have 

 been made out (they need be only mentioned here) are chiefly four: by 

 embryological study of the paths, which develop at different times; by 

 the observation of their degeneration when cut, away from their trophic 

 central cells; by observation of pathological cases (tumors, stabbings, 

 etc.); and by direct vivisectional experiment on animals similar to man. 

 How large the number of tracts through the white matter from the 

 numberless functional regions of the body we have as yet no means of 



