ANIMAL BIOLOGY. 



[Part I. 



To see this central nervous system the frog should be turned 

 over and pinned out back upwards. An incision should be made 

 in the skin, and the muscles cleared away from the vertebral 

 column. The dorsal cavity should then be opened out, begin- 

 ning at the junction of the skull and vertebral column, and 

 removing the roof of the skull and the upper portions of the 

 vertebrae. The brain and spinal cord will thus be displayed. In 



FIG. 12. BRAIN OF FROG. 

 A. From above. B. From below. C. In longitudinal section. 



the brain three parts will readily be seen : a, fore-brain, 1 composed 

 of two cerebral hemispheres (12, c. h.) lying side by side, and 

 passing anteriorly into the olfactory lobes (olf.), which are joined 

 in the mid-line; a mid-brain, composed largely of two large 

 rounded optic lobes (op. I. ) ; and a hind-brain, composed of a band- 

 like cerebellum (cb.), and behind this the medulla oblongata (m. 0.), 

 the upper surface of which is marked by a triangular depression, 

 covered over by a plexus of blood-vessels. Dissection will 

 readily show the cerebral hemispheres and optic lobes to be 

 hollow. 



The spinal cord or myelon is continuous with the medulla 

 oblongata; it presents two enlargements where the nerves 

 (1) for the arms, and (2) for the legs, are given off. It ends in 

 a fine point, ihefilum terminale. 



6. The Skeletal System. Somewhat has been said of this 



1 The primitive fore-brain and its outgrowths may be regarded as forc-l>r<iin. 

 See Chapter XL 



