THE PERCH 47 



surface. At its anterior end will be seen the small hemispheres, 

 with the olfactory lobes at their anterior ends. Back of them is 

 the diencephalon, from which project the following structures: 

 (i) the optic nerves with the optic chiasma and (2) the median 

 infundibulum, at the lower end of which is the hypophysis or pitu- 

 itary body and at whose sides are a pair of large swellings called 

 the lobi inferiores. At the side of each of these swellings on the 

 ventral surface of the brain the oculomotor nerve arises, while 

 posterior to them is the medulla oblongata. Note the median fur- 

 row in it, and the roots of the delicate abducens near the median 

 line. Identify the roots of the other cranial nerves. 



Exercise 15. Draw the ventral aspect of the brain on a scale of 3 or 4. 



Cut the brain in two in exactly the sagittal plane and study 

 the cut surface. The brain is a hollow structure ; in it is a 

 series of cavities which are a continuation of the central canal 

 of the spinal cord. The most posterior of these cavities, which 

 is directly continuous with the central canal, is in the medulla, 

 and is called the fourth ventricle ; the cerebellum projects partly 

 over it. Note the thick ventral wall and the thin dorsal wall of 

 this ventricle. Note also that the cerebellum is solid and that 

 beneath it the fourth ventricle continues forward between the 

 optic lobes, forming a canal called the aqueductus Sylvii; this 

 canal goes to the diencephalon, where it is called the third ventricle. 



The optic lobes are hollow, the cavity in them communicat- 

 ing with the aqueductus Sylvii. On the floor of each lobe is a 

 crescent-shaped ridge called the torus. A W-shaped median fold 

 of the posterior wall of the lobes will be seen between the two 

 tori, and in front of it a median fold of the anterior wall. Care- 

 fully remove the thin dorsal wall of one of the lobes and study 

 these structures. 



In the two hemispheres are the first and second, or lateral, ven- 

 tricles, which are shallow spaces communicating with the third 

 ventricle. Make a transverse section of one of the hemispheres 

 and note the floor of its ventricle ; this is much thickened and is 

 called the corpus striatum. 



Exercise 16. Draw the sagittal section on a scale of 3 or 4. 



