NERVOUS TISSUE. 187 



segments of different material arranged in regular order ; 

 S — sz — IS — sz — S — sz — IS— sz — S : S representing the 

 septal line ; sz, the septal zone ; IS, the inter-septal zone. 

 Of these, IS is the chief if not the only seat of the 

 mj'osin; what the composition of sz and of S may be 

 is uncertain, but the supposition, that, in the living 

 muscle, sz is a mere fluid, aj^pears to me to be wholly 

 inadmissible. 



When living muscle contracts, the inter-septal zones 

 become shorter and wider and their margins darker, 

 while the septal zones and the septal lines tend to 

 become effaced — as it appears to me simply in conse- 

 quence of the approximation of the lateral margins of 

 the inter-septal zones. It is probable that the sub- 

 stance of the intermediate zone is the chief, if not the 

 only, seat of the activity of the muscle during con- 

 traction. 



5. The elements of the nervous tissue are of two kinds, 

 nerve-cells, and nerve fibres ; the former are found in the 

 ganglia, and they vary very much in size (fig. 54, B). Each 

 ganglionic corpuscle consists of a cell body produced 

 into one or more processes which sometimes, if not 

 always, end in nerve fibres. A large, clear spherical 

 nucleus is seen in the interior of the nerve-cell ; and 

 in the centre of this is a well defined, small round 

 particle, the nucleolus. The corpuscle, when isolated, 

 is often surrounded by a sort of sheath of small nucle- 

 ated cells. 



