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 
myosin; 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, appears 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. 
