THE NERVOUS SYSTEM. 187 



arachnoid is the subdural space, containing a fluid having 

 the nature of lymph. The arachnoid is a very delicate 

 membrane which does not dip down into the sulci between 

 the convolutions of the brain, as is the case with the 

 pia mater, but passes across these depressions, where it 

 may be easily demonstrated (Fig. 100). 



The subarachnoidal space lies between the arachnoid 

 and pia mater. The pia mater is the delicate vascular 

 membrane following so closely the convolutions and sulci 

 of the brain that it becomes apparent only when lifted 

 carefully by the forceps. 



In order to study the brain satisfactorily each student 

 should have a specimen hardened according to directions 

 in the section on technique, and also should be permitted 

 to examine a series of sections cut transversely about a 

 centimeter thick. These sections may be mounted for 

 permanent use in pasteur dishes, according to the method 

 described in the "Journal of Applied Microscopy," Octo- 

 ber, 1902. The brain of a calf or sheep serves the learner's 

 purpose better than that of a cat, as the parts are larger, 

 and they are also easier to procure. An additional ad- 

 vantage is also found in the fact that there is some 

 difference between the brains of the cat and calf, and 

 the student must therefore rely more on his own observa- 

 tions. 



External Features.- -The brain is composed of five 

 parts : the medulla oblongata, or myelencephalon; the pons 

 Varolii and cerebellum, forming the metencephalon; the 

 corpora quadrigemina and crura cerebri, composing the 

 mesencephalon; the diencephalon, including the optic 

 thalami and other parts bounding the third ventricle; 

 and the telencephalon, or cerebral hemispheres. The 

 medulla oblongata, sometimes called the bulb or stem 

 of the brain, is the expansion of the spinal cord as it 



