CHAPTER V 



The Central Nervous System 



ALLOW a frog to lie for a day or two in methylated spirit. Remove 

 the muscles from the surfaces of the arches of the spinal column. 

 Open the neural canal by dividing the membrane between the skull 

 and the vertebrae. Cut away, bit by bit, the bony roof of the skull 

 by means of a narrow-bladed but strong pair of scissors. Remove 

 the arches of the vertebrae in the same way. The spinal cord is 

 usually concealed more or less by soft material, which can be gently 

 removed with a blunt pair of forceps. 



Notice the brain in the skull, the spinal cord in the vertebral 

 column, continuous with one another. 



The Brain. In front, two elongated masses forming half of the 

 brain. Each has an anterior small part and a large posterior part. 

 The former is the centre for smell (olfactory lobes'] ; the latter, the 

 cerebral hemispheres ; on their surface the cortex. 



Between the posterior ends of the cerebral hemispheres the 

 thalami appear, and from them an optic lobe (the mid-brain), 

 forming a rounded eminence, projects on each side. 



A narrow band, the cerebellum, lies transversely just behind the 

 optic lobes. It is very small in the frog. 



The rest of the brain forms the bulb, and contains very important 

 nerve centres. Its dorsal aspect presents a triangular hollow, the 

 fourth ventricle. 



Raise the brain carefully, beginning anteriorly and cutting 

 through the nerves passing from it through the skull. Notice the 

 ventral aspect of the structures mentioned above. 



In the spinal cord note the number of nerves given off, and that 

 each arises by two roots. The dorsal root contains the afferent 

 or sensory fibres, the ventral root the efferent or motor fibres. 



Sympathetic System. Turning the frog on its back, remove 

 the viscera and find the sympathetic trunk on each side of the main 

 arterial trunk, the aorta. It is a slender cord with ganglia (enlarge- 

 ments containing nerve cells) at intervals. Note the delicate nerves 

 uniting these ganglia to the spinal nerves, most easily seen in the 

 case of the long nerves to the hind-legs. 



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