146 HOW TO WORK 



caused Schroeder, Van dcr Kolk, and some others, to abandon the 

 method at first, but if the sections be set aside for a little while, 

 and treated occasionally with a little turpentine, the cells and fibres 

 reappear, and present a beautiful appearance. Before they are 

 finally covered with thin glass, they should be examined at intervals 

 under the microscope, to see whether all the details of structure have 

 come out dearly ; and if so, as much Canada balsam must be used 

 as suffices for mounting. If the sections be of considerable thickness, 

 it will be found best to place them in a shallow vessel, the bottom of 

 which is kept simply wet with turpentine, which can therefore ascend 

 through them from below, while the spirit evaporates from their 

 upper surfaces, for the principle of the method is this : to replace 

 the spirit by turpentine, and this by Canada balsam, without drying 

 the sections. The method at first is attended with some difficulty, 

 and practice is necessary to ensure complete success. Experience,, 

 also, may suggest, according to circumstances, certain modifications 

 of the exact process here given, which, to a certain extent, must be 

 considered as general. 



This method is now generally adopted in investigating" the structure 

 of the brain and spinal cord. Longitudinal and transverse sections 

 of the spinal cord are represented in pi. XXXIV, figs. 220, 221. 



245. Examination of Nerve Ganglia. The sympathetic ganglia 

 and the ganglia on the posterior roots of the nerves should be 

 obtained from young animals, for in adults and in those advanced in 

 age, the quantity of connective tissue is so great as to hide many of 

 the cells and render it impossible to trace for any great distance the 

 very pale delicate nerve-fibres connected with them. In investigations 

 upon the structure of these cells I have pursued the plan of inves- 

 tigation described in part V, by the aid of which I was enabled to 

 demonstrate that at least two fibres (one of which in the frog's 

 ganglion cells was coiled round the other) came from every one of 

 these ganglion cells, and that the fibres when they reached the nerve 

 trunks pursued opposite directions. See pi. XXXIV, fig. 223. 



OF COLLECTING, KEEPING ALIVE, AND EXAMINING THE 

 LOWER ANIMALS. 



246. Of Collecting and Dredging. To those fond of natural his- 

 tory, few things are more delightful than a ramble over the beach at 

 low water for the purpose of collecting. Sea dredging adds not a 

 little to the charms of boating, and by the aid of the dredge many 

 interesting creatures may be caught, which never advance to low 

 water mark. But there are many organisms which inhabit shallow 



' fresh water pools of great interest to the observer. The apparatus 



