766 TEE CENTRAL AXIS OF TEE NERVOUS SYSTEM 



is more diflScnlt than the first, bnt possesses several advantages over it; for not only do we 

 preserve the pituitary gland, but have the ethmoidal lobes more intact, and may also have, 

 if desired, the ganglia of the cranial nerves, with a more or less considerable portion of the 

 nerves themselves. 



After indicating the methods for extracting the brain from its bony case, we ought to say 

 some words as to tlie course to be pursued in order to study it successfully. To do this it is 

 advantageous to have two brains ; one of these should be hardened by steeping it for some 

 weeks in alcoliol (or methylated spirit), or in water to which has been added a tenth part of 

 nitric acid. This hardening contracts the nerve substance, and causes the cavities and reliefs 

 to appear more manifest. (It is a good plan to place the brain, base uppermost, in a suitable 

 vessel, and if a piece of cloth be spread beneath it, its removal therefrom will subsequently be 

 greatly facilitated.) 



We commence by examining rapidly the whole apparatus, and pass immediately to the study 

 of the isthmus, of which it is necessary to have at first a well-defined idea. We therefore take 

 a hardened specimen, and isolate this portion of the brain in the manner represented in Fig. 

 425 ; to do this, it suffices to cut through tlie peduncles of tbe cerebellum, and excise the cere- 

 bral liemispheres upwards and backwards; the remains of these and the cerebellum should be 

 preserved for an analysis of their structure. The isthmus thus isolated is fitted for an examina- 

 tion of its external conformation and its internal cavities — the ventricle of the optic layers and 

 the aqueduct of Sylvius — into which we may penetrate by a superior longitudinal incision. 



After the isthmus, the cerebellum is to be studied — in its external conformation — on an 

 intact specimen ; and in its internal conformation and structure on the incised piece. 



We terminate with the cerebrum, the superficies of which is soon examined ; its interior 

 should be studied in the following manner: It is necessary to begin by demonstrating the 

 . existence of ventricles in the olfactory lobes, and their communication with all the other in- 

 ternal cavities of the brain, which can easily be done by the inflation of one of these organs by 

 means of a straw (or dissecting-case tube), which raises the pituitary gland, the cerebrallobes 

 and the cerebellum. Tiien we pass to the corpus callosum, which is exposed, as in Fig. 434, 

 by a horizontal section of the hemispheres across the centrum ovale. The corpus callosum of 

 each side is afterwards excised in the middle line to reach the interior of the lateral ventricles, 

 and this great commissure of the brain ought, after studying the septum lucidum, to be cut 

 across in the middle and turned over, as in Fig. 435, so as to show the fornix. The foramen 

 of Monro is next examined; then the corpus striatum, hippocampi, taenia semicircularJB, 

 choroid plexus, and velum interpositum. which are exposed by the ablation of the hippocampi 

 and fornix Lastly, we return to the foramen of Monro to study its communication with the 

 ventricle of the (jptic thalanii ; it will be well, also, to again examine the latter, as well as the 

 aqueduct of Sylvius and the ventricle of the cerebellum, which we arrive at in dividing 

 that organ through the middle and separating the halves. 



Two longitudinal and vertical sections, one median (Fig. 428), the other at the side (Fig. 

 431), will not be without utility in the study of these particulars. They may be made by 

 means of a saw, the brain remaining enclosed in the cranial cavity. 



(A useful implement I have employed for removing the bony casing of the brain without 

 risk of injuring the latter, is a chisel with a thin cutting edge which is slightly concave, the 

 corners being smooth and rounded, and projecting beyond the cutting edge.) 



The brain may be preserved for an indefinite time when it has undergone the following 

 preparation : 1. Place it for from twelve to fifteen days in a solution of nitric acid (10 to 100). 

 2. Then leave it for a week in a solution of bichromate of potassium (20 to 1000). 3. Afterwards 

 sleep it for three or four days in alcohol at 40° 4. Take it from the alcohol, allow the latter 

 to evaporate, then put it in a paraffin bath for from ten to fifteen minutes, not allowing it to boil. 

 5. Withdraw it from the paraffin and allow it to dry. After this series of preparations the 

 brain shrivels, but it preserves its shape, and will not putrefy. 



Article II. — The Isthmus (or Medulla Oblongata). 



We will study in succession the external and internal conformation of this 

 part, and its structure. 



External Conformation of the Isthmus. 



The isthmus, or medulla oblongata, is a prismatic prolongation of the spinal 

 cord supporting the cerebellum, and terminating in the cerebral hemispheres ; 



