THE MAMMALIAN NERVOUS SYSTEM 57 



commonly referred to, as in the preceding paragraphs, as forming 

 the true roof of the brain cavity in these regions. But according 

 to the more precise usage of the B NA each of these structures 

 is really composed of two layers: (1) the lamina epithelialis, 

 which is the true brain wall derived from the embryonic neural 

 tube, which here retains its embryonic character as a non- 

 nervous epithelium; (2) the pia mater, which is here highly vas- 

 cular and convoluted. In the BNA the term plexus chorioideus 

 is applied to this modified pia mater alone. 



3. General Directions for Microscopic Material 



61. The further study of the spinal cord and brain stem can 

 best be carried out with both gross and microscopic material, 

 though good results can be obtained with either class of material 

 alone. If microscopic preparations are not available, it will be 

 found practicable to select from the following paragraphs those 

 which apply to gross material only. Gross sections through the 

 human brain at the levels indicated below will show many of the 

 details referred to. 



62. Drawings of cross-sections. — Make an outline sketch, 

 ventral side down, of each of four microscopic transverse sections 

 (Weigert method preferred) through the spinal cord taken re- 

 spectively from the cervical, thoracic, lumbar, and sacral regions; 

 and also select 6 to 12 transverse sections from the series through 

 the brain stem and similarly draw each of these in outline. 

 Make these outlines as accurately as possible about six times 

 natural size, or those from the spinal cord and lower part of the 

 medulla oblongata may be magnified 8 diameters and those from 

 the upper levels 4 diameters. The sketches of the sections 

 should include the outlines of a few only of the more important 

 features, such as the inferior olives, to serve as points of refer- 

 ence. The following levels are recommended: 



(1) Upper cervical cord. See Bruce ('92), Fig. 1, Plate XIII; 

 Bruce ('01), Plates I-VIII; Cunningham ('15), Fig. 466; Piersol 

 ('13), Fig. 895, p. 1041; Rauber-Kopsch ('07), Fig. 634, p. 553. 



(2) Lower part of decussation of pyramidal tracts. See 

 Bailey ('16), Fig. 333, p. 490; Bruce ('92), Plate III; Cunning- 

 ham ('15), Fig. 490, p. 558; Piersol ('13), Figs. 920, 921, p. 1069; 



