THE SOMATIC NERVOUS SYSTEM. 67 



which the stroma is more apparent, except in the greater 

 abundance of the infiltrating fatty material. 



Dietl observes that the medullated stroma onl}' resembles 

 the white matter in vertebrates (the medullary sheath of 

 Schwann) in its reaction with osmic acid. In this I am not 

 inclined to agree with him ; it appears to me that the interstitial 

 substance differs but little from the white substance of Schwann, 

 and chiefly in not being differentiated into a distinct sheath 

 around each fibre. I regard it as an interfibrillar substance, or 

 matrix, in which the axis cylinders, whether bundles of larger 

 fibres or fine stroma are imbedded. I shall distinguish the two 

 forms of stroma as non-medullated and medullated stroma. 



The Gray or Cortical Substance consists either of large or small 

 ganglion cells. 



The large ganglion cells are stellate with numerous branch- 

 ing processes, which are continuous with the stroma of the 

 central white substance. Some of the processes are directly 

 continuous with the nerves, but many of the nerve-fibres of the 

 peripheral nerves arise from the stroma. 



The small ganglion cells are chiefly found in the supra- 

 oesophageal centres, and form a layer many cells thick. They 

 are spherical nuclear corpuscles, surrounded by a very little 

 protoplasmic cell substance, which is prolonged in a fine fibre, 

 and unites these cells in strings or chaplets, ultimately becom- 

 ing continuous with the central stroma. The small round cells 

 resemble those of the nuclear layers of the cerebellum and 

 retina in vertebrates. 



The external capsule, or peritoneal coat, varies greatly in 

 thickness ; but differs in no way from the mesoblastic tissue 

 covering the other internal organs. 



c. The Somatic Nervous System and the Neuroblast of the 

 Blow-fly Larva. 



The central nervous system in the larva of the cycloraphic 

 Diptera generally, differs widely from that of other insects in 

 the close concentration of all the ganglia in a single complex 

 centre, which consists in part of the differentiated nervous 



