M . A '1' b:R I A 1, A S I) METHODS
The material f o r the present iiivestigation was a s follows : a brain stem from a human fetus, stillborn a t ahout 7 months, stained in hulk with Kansoii 's pyridin-silver process and sectioned seriall>-; ail adult human brain stem sectioncd serial1:-in N transverse direction and stained by the Pal-Weigert method ; a11 adult sheep brain stem sectioned wrially in a transverse directioii and stained bj-the Pal-7Veiger.t method ; two opossiim brains and thc medulla of an adult cat stained with the pyridiii-silver process a i d sectioiied serially. Thew were compared with material from the cat, dog, rahhit a n d rat prcvionsly described. The whole h a i n stems inchided generous portions of the cerebral hemispheres i n each case. Rectioiis of the 1%-JTcigert material were 50 and 75 11 in thickness, wliil(> tlic pyritlin silver preparations were 15 p. Ohserrations wcrc made with a nioii-ohjeclivc binocular microscope eqnippecl with apochromatic lenses, the immersion ive having a numerical aperture of 1.40.
IITSTORIrAL H E V I E W Numerous his t ologica 1 descaript ion s of tl i ti iiirierva tioii of the pia mater and its vessels have appearc.d from the time of Purkinje's (1845) observations nntil the preheiit day, hut