144 THE GROWTH OF THE BRAIN. 



into the brain-stem. Next in size come the large pyra- 

 midal cells in the cerebral cortex, the cells of Purkinje 

 in the cerebellar cortex and the great ovoid cells in the 

 spinal ganglia. Besides these there are to be found a 

 host of smaller forms everywhere, except in the spinal 

 ganglia, where very small cells do not occur. Fig. 20 

 represents a fully-grown large nerve cell from the ventral 

 horn of the spinal cord of man. This may be taken as 

 the basis for a more extended description. 



The description of the nerve cell begins with the 

 nucleus, since that structure is the first to be clearly 

 developed. In the early stages the nucleus is ovoid and 

 contains a chromatic or deeply staining substance in 

 small masses. Representing at the start almost the 

 entire cell, the nucleus becomes later surrounded by 

 what is really an enormous quantity of cytoplasm, and 

 also forms within itself one, or even two, inclusions. The 

 larger of these, the nucleolus, is deeply stainable, and this 

 in turn may develop the second inclusion, the nucleololus, 

 much more tiny than itself, and noticeable only in the 

 largest cells. It is the cytoplasm, of course, which, 

 according to the locality, may be pyramidal or ovoid in 

 form. Perhaps the most noticeable substance in the 

 cytoplasm is the pigment. As a rule this is not present 

 at birth, but appears later, and increases with age. 



A stainable substance is also found distributed in the 

 cytoplasm and in all its prolongations, except the one 

 designated the axis-cylinder process or the neuron : thus 

 this process even at its base exhibits a character diffe- 

 rent from that of the other prolongations. By some 

 methods of preparation the granules in the cytoplasm 

 appear to be arranged in rows ; these rows are some- 

 times concentric with the nucleus, but near the periphery 

 of the cell they often extend into the prolongations, and 

 if a significance is attached to their arrangement on the 



