I7 6 ELEMENTS OF MAMMALIAN ANATOMY. 



The subarachnoidal space lies between the arachnoid and 

 pia mater. The pia mater is the delicate vascular mem- 

 brane following so closely the folds and clefts 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 

 on page 10, and also should be permitted to examine a 

 series of sections cut transversely about a centimeter thick. 

 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 advantage 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 observations. 



External Features. The brain is composed of five 

 parts : the medulla oblongata, or inyelencephalon; 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 passes through the 

 foramen magnum. The pons Varolii is the bridge of 

 transverse fibers seen on the ventral aspect of the brain just 

 craniad of the medulla (Fig. 93). The cerebellum or 

 little brain lies on the dorsal side of the medulla and is 

 partly covered by the caudal part of the cerebrum. The 

 mesencephalon is not visible externally on the dorsal aspect, 

 but may be seen immediately craniad of the cerebellum by 

 cutting away the cerebrum (Fig. 96). The diencephalon 

 is visible externally only on the ventral aspect of the uncut 

 brain, where it forms the floor of the third ventricle. It 



