478 THE ORGANS 



(2) For the study of the general internal structure of the cord, stain a section 

 through the lumbar enlargement of a cord prepared according to the preceding 

 technic (i) in haematoxylin-picro-acid-fuchsin (technic 3, p. 21) and another 

 section through the same level in Weigert's ha^matoxylin (technic p. 33). Mount 

 both in balsam. For Weigert staining, material fixed in formalin or in Orlh's 

 fluid should be further hardened in Miiller's fluid for at least a month, changing 

 the fluid frequently at first to remove the formalin. Mallory's glia stain should 

 also be used with material fixed in Zenker's fluid (technic, p. 30). The silver 

 method of Cajal (alcohol-fixation) should also be used (technic, p. 38) and that 

 of Nissl. 



(3) From a cord prepared according to technic i, remove small segments 

 from each of the following levels: (i) the twelfth dorsal, (2) the mid-dorsal, and 

 (3) the cervical enlargement. The segments are embedded in celloidin, sections 

 cut 15 to 2o;£ thick, stained by Weigert's method (page TiS), and mounted in 

 balsam. McduUated sheaths alone are stained by this method and appear 

 dark blue or black. 



(4) A human cord from a case in which death has occurred some time after 

 fracture of the vertebrae with resulting crushing of the cord, furnishes valuable 

 but of course rarely available material. If death occur within a few weeks after 

 the injury, the method of Marchi should be used ; if after several weeks, the method 

 of Weigert (page :i^). The picture in the cord is dependent upon the fact that 

 axones cut off from their cells of origin degenerate and disappear. After a com- 

 plete transverse lesion of the cord, therefore, all ascending tracts are found de- 

 generated above the lesion, all descending tracts below the lesion. The method 

 of Marchi gives a positive picture of osmic-acid -stained degenerated myelin in 

 the affected tracts. The method of Weigert gives a negative picture, the neu- 

 roglia tissue which has replaced the degenerated tracts being unstained in con- 

 trast with the normal tracts, the myelin sheaths of whose fibres stain, as usual, 

 dark blue or black. 



(5) Human cords from cases which have lived some time after the destruc- 

 tion of the motor cortex, or after interruption of the motor tract in any part of 

 its course, may also be used for studying the descending fibre tracts. 



(6) The cord of an animal may be cut or crushed, the animal kept alive for 

 from two weeks to several months, and the cord then treated as in technic (4). 

 The most satisfactory animal material may be obtained from a large dog by 

 cutting the cord half-way across, the danger of too early death from shock or 

 complications being much less than after complete section. 



(7) The cord of a human foetus from the sixth month to term furnishes good 

 material for the study of the anterior and posterior root fibres, the plexus of 

 fine fibres in the gray matter, the groupings of the anterior horn cells, etc. The 

 pyramidal tracts are at this age non-meduUated and are consequently unstained 

 in Weigert preparations. The cord of an infant of about one month is also ex- 

 cellent. Here the majority of the pallio-spinal fibres are medullated but very 

 thinly so that they are easily distinguishable by their lighter stain. The Wei- 

 gert-Pal method gives the best results. 



(8) For the study of the course of the posterior root fibres within the cord, 

 cut any desired number of posterior roots between the ganglia and the cord 



