ANATOMY AND PHYSIOLOGY 23 



branes? What happens when the blood supply of the human brain is 

 reduced or shut off in limited areas? 



(e) The brain and spinal cord are to be studied in place, but the 

 membrane covering them should now be removed in order that the 

 parts of the central nervous system can be seen more clearly. Be- 

 ginning at the anterior end, the following parts of the brain can be 

 identified as it lies in place: (1) The cerebral hemispheres, or cere- 

 brum, a pair of elongated, nearly cylindrical structures, from the an- 

 terior ends of which the pair of olfactory lobes is indistinctly separated. 

 (2) The diencephalon, a somewhat depressed region, the top of which 

 may be covered by a very vascular, thin roof, the anterior choroid 

 plexus; if this has been removed, the cavity of the third ventricle will 

 be seen. (3) The optic lobes, a pair of prominent rounded bodies. (4) 

 The cerebellum, a narrow transverse ridge just posterior to the optic 

 lobes. (5) The medulla oblongata, part of the roof of which is thin, 

 very vascular, and known as the posterior choroid plexus. This covers 

 the fourth ventricle, which will be exposed by the removal of the choroid 

 plexus. The medulla is the most posterior region of the brain and is 

 continuous with the spinal cord. The spinal cord is not uniform in 

 diameter throughout its length. Observe the slightly enlarged anterior 

 brachial region and posterior lumbar region. What groups of spinal 

 nerves arise in these regions? Beyond the lumbar region the spinal 

 cord tapers rapidly and ends in the filum terminale. 



(f) Ten pairs of cranial nerves arise from the brain, and ten 

 pairs of spinal nerves arise from the spinal cord in the frog 

 (Fig. 22). Many of these nerves are too small to be seen easily 

 but certain ones can be identified. Four pairs of cranial nerves can 

 usually be found. The olfactory nerves, or first pair of cranial nerves, 

 can be exposed by cutting away the bone immediately in front of the 

 olfactory lobes; they are short nerves running to the nasal cavities. 

 If they were not destroyed during the removal of the eyes the optic 

 nerves, or second pair, can be seen by gently pressing the brain away 

 from the side of the skull; they arise from the ventral surface of the 

 brain. The trigeminus nerves, or fifth pair, arise from the anterior 

 end of the medulla on each side and run to the face. The auditory 

 nerves, or eighth pair, can usually be found near the fifth and run 

 to the inner ear. Carefully push the spinal cord away from the 

 vertebrae and find the roots, or places of origin, of the spinal nerves 

 studied in Exercise 19. Draw the brain and spinal cord (X 3) as seen 

 from dorsal view. Indicate the positions of the nerves you have 

 found. 



