360 THE NERVOUS SYSTEM 



To which part does the pineal body belong? The hypophysis? Locate the quadri- 

 geminal lamina, cerebral peduncle, cerebellum, and medulla oblongata. 



23. Now turn the specimen over and carefully dissect away what remains of the 

 skin and mesodermal tissues so as to expose the brain and cranial nerves from the lateral 

 side. Identify all the parts labeled in Fig. 259. 



GENERAL TOPOGRAPHY OF THE BRAIN 



24. The adult mammalian brain should now be compared with that of the shark 

 and with that of the fetal pig. If two sheeps' brains are available, one should be divided 

 into lateral halves by a cut made exactly 1 mm. to the left of the median sagittal plane. 

 Use a long, thin brain knife and make the cut with a single sweep. Put away the right 

 half for future study. On the left half and on the intact brain identify all of the chief 

 divisions of the brain, determine their embryologic derivation, and compare them 

 with similar parts in the brains of the shark and fetal pig. (See the table on p. 36, 

 pp. 113-116, and Figs. 82-84.) 



25. By a study of the medial aspect of the left half of the brain ascertain what 

 relations the various subdivisions bear to each other. (See Fig. 84 and pp. 116-118.) 

 Note the difference in color between the cortex and the white center of the cerebellum. 

 By tearing away the cerebellum a little at a time make a dissection of the cerebellar 

 peduncles on this half of the brain (Figs. 87, 91). Scrape away the superficial gray 

 matter from the rostral end of the left hemisphere and uncover the white substance 

 beneath. The superficial gray matter is known as the cerebral cortex and this covers 

 the white center of the cerebral hemisphere. 



NEUROLOGIC STAINS 



26. Some knowledge of how various stains act on the nervous tissues is essential 

 for an understanding of the special preparations which are to be studied. The technic 

 involved in preparing such material is described in books devoted to technical methods 

 (Hardesty, 1902; Guyer, 1917). 



27. Osmic Acid. Small nerves may be fixed in osmic acid. This stains the myelin 

 sheaths black. Why? Axons remain unstained. 



28. The Weigert or Pal-Weigert Method. When a portion of the brain or spinal 

 cord has been treated for several \veeks with a solution containing potassium bichromate 

 (Miiller's fluid) the myelin sheaths acquire a special affinity for hematoxylin, by 

 virtue of which they become deep blue in color when stained by this method. Axons, 

 nerve-cells, and all other tissue elements remain colorless unless the preparation has 

 been counterstained. The method is adapted for the study of the development and 

 extent of myelination and for tracing myelinated fiber tracts. This method may also 

 be used for a study of degenerated fiber tracts, which remain colorless in preparations 

 in which the normal fiber tracts are well stained. 



29. The Marchi method is a differential stain for degenerating fibers. These 

 contain droplets of chemically altered myelin. The tissue is fixed in a solution contain- 

 ing potassium bichromate (Muller's fluid). This treatment prevents the normal 

 myelinated fibers from staining with osmic acid, but does not prevent the droplets of 

 chemically altered myelin in the degenerated fiber from being stained black by this 



