216 INTRODUCTION TO GENERAL PHYSIOLOGY 



Make a similar preparation of a mammal, such as a rat. Note 

 that the cerebral hemispheres and the cerebellum have grown so 

 much as to cover over the other parts, which can only be seen by 

 raising or removing the former. The optic lobes, on the other 

 hand, are relatively small, and are represented in the mammal by 

 the corpora quadrigemina, four little eminences. 



Examine in a museum the remains of various extinct verte- 

 brates, such as the Plesiosaurus. Note the small size of the skull 

 in proportion to the rest of the body. Compare the state of affairs 

 in the cat or man, and realise that the law of progress is that the 

 prizes go to the wise, and that the main factor in evolution is the 

 development of the nervous system. 



The cell-bodies of the spinal neurones are well seen by taking a 

 piece of the fresh spinal cord of the ox. With the point of a 

 scalpel, take out a small piece of the central grey matter from the 

 more ventral broader part Tease in 0.9 per cent, saline and put 

 on a cover-glass, with a bit of hair to protect the cells from pressure. 

 Look for large branching cells. 



Another way by which fixed cells are obtained is to allow a 

 small piece of the grey matter to macerate for a day or two in very 

 dilute chromic acid in a small stoppered bottle. Shake. Allow 

 to deposit, and mount some of the deposit as above. This prepara- 

 may be stained and preserved by mounting in dilute glycerine. 



Similar preparations should be made of the cortex of the 

 cerebral hemispheres and of the cerebellum of the rat. 



Spinal Reflexes 



Some observations were made above (p. 209) on the spinal frog. 

 A more detailed study is to be made here. 



Having made the preparation as before, hang up the frog to 

 the edge of the table by a pin through its jaw. 



Gently pinch one of the toes. Note the movement. 



Apply a very small bit of filter-paper dipped in acetic acid to 

 the skin of one flank or thigh. The leg will wipe it away. 



Stimulate the toes of one side by a stimulus that can be adjusted 

 in strength, such as the induced currents from the induction coil. 

 Note that the number of muscles engaged in the reflex increases 

 with the strength of the stimulus. Also that the time the move- 

 ments last after the stimulus has ceased is longer with strong 

 stimuli. 



Dip the foot in weak sulphuric acid of progressively increasing 

 strength (o.i, 0.2, 0.3, 04, and 0.5 per cent), dipping the foot in 

 water as soon as the reflex has been observed, Note that there 



