Metazoa Rana. 3 1 3 



from its appearance under the naked eye, been called 

 grey matter ; when the nerve-fibres are more abundant, 

 or alone present, the nervous tissue is known as white 

 matter. In both white and grey matter there is a consider- 

 able amount of connective tissue (neuroglcea) present. 



A section of the cerebral hemisphere, or cerebellum, 

 shows it to be composed externally of a layer- of grey matter 

 that is, of nerve-cells and neuroglcea and internally of 

 white matter that is, of nerve-fibres and neurogloea. The 

 cerebellum, optic lobes, and other parts of the brain pre- 

 sent the same arrangement of elements. A careful examina- 

 tion of the sections of the frog's brain shows that there is a 

 complicated series of spaces and channels in the interior, all 

 of which are continuous with a canal which runs down the- 

 centre of the spinal cord. 



If a section of the spinal cord be now examined the 

 arrangement of elements will be found to be somewhat dif- 

 ferent. Here the grey matter occupies the interior and 

 has roughly the form of an H, the central canal just alluded 

 to piercing the middle of the cross-bar. The grey matter is 

 said to have two anterior and two posterior horns, and if 

 the section be taken at the origin of a spinal nerve it will 

 be found that the two roots of the nerve are connected 

 with the anterior and posterior horns respectively. The 

 central canal is lined by ciliated epithelium. Surround- 

 ing the grey matter on all sides lies the white matter of the 

 cord, consisting of nerve-fibres and connective tissue and 

 surrounded in turn by the sheath of delicate connective 

 tissue largely supplied with blood- vessels, which is continuous 

 over the entire brain and spinal cord, and which is known 

 as the pia mater. The pia mater dips down into the cord 

 anteriorly and posteriorly almost to the transverse band of 

 grey matter, thus nearly dividing the cord into two parts. 

 These two clefts are known as the anterior and posterior 

 fissures, and it is the mouth of the posterior fissure that 

 gives the grooved appearance to the dorsal aspect of the 



