METHODS OF RECONSTRUCTION. 387 



complicated reconstructions, the student is referred to figures 169 and 172, pages 

 229 and 235. 



Oftentimes simpler reconstructions are very helpful in which only a few sections 

 are combined, as, for example, to show the course and branches of the spinal 

 nerves in young embryos. In such a case the outline of the middle section of 

 the series proposed to be combined may be selected to give the outline of the 

 reconstructed drawing. Camera lucida drawings of this and the neighboring sec- 

 tions to be included should be made of the desired magnification. The reconstruc- 

 tion itself may be made upon tracing paper, which is laid successively over the 

 drawings of the sections and the parts required from each can be added upon the 

 tracing paper, which will thus combine in a single drawing the parts intended to 

 be represented. Reconstructions of this kind are easily made by students and are 

 often very instructive. 



Reconstruction with Wax Plates by Barn's Method. The basis of this method 

 is to make in wax a magnified reproduction of the single sections, representing in 

 the wax such portions of the section as it is desired to reproduce in plastic recon- 

 struction. To this end wax plates must be made which represent a definite magni- 

 fication of the thickness of a section. For working by this method it is usually 

 advantageous to employ rather thick sections, say, of 20^. If the magnification 

 chosen is fifty times, which is practically often convenient, then the wax plates 

 should be made fifty times 2o/i in thickness, or i mm. The most convenient 

 plates to work with are those from i to 2 mm. thick. Upon a wax plate of the 

 requisite thickness a camera lucida drawing is made. This may be done with a 

 lithographic crayon or with a fine steel point. The drawings must be of exactly 

 the right magnification; in the illustration chosen, 50 diameters. Next, the wax plate 

 is put upon a glass or a metal surface where it lies perfectly flat, and with a 

 sharp thin-bladed knife or scalpel the outlines of the organs which it is intended 

 to reconstruct are cut out as may be desired. Our bit of wax then represents a 

 model of the parts selected from the section, and equally magnified in the three 

 dimensions of space. Wax plates made from successive sections are then piled 

 up, one on top of the other, in the proper order. If they are rightly superimposed, 

 an operation which often requires skill and judgment, and always requires the 

 utmost care, then the pile of plates will correctly represent the form of the parts 

 included in the reconstruction. To fasten the plates together it is only necessary 

 to pass a warm metal instrument over the edges of the plates, enough to melt the 

 wax a little. With proper care this may readily be accomplished without destroy- 

 ing the surface modeling of the reconstruction. 



The simplest method of making wax plates is to have a large tin pan with 

 vertical sides. This is filled with very hot water, and melted beeswax is poured on 

 the surface of the water and allowed to cool. Plates of sufficiently exact and even 

 thickness may be cast in this way, provided the operation is carried out in a quiet 

 place so that the surface of the water is not disturbed while the wax is hardening. 



