Houser, Zhe Neurones of a Selachian. 89 
ber, arising from the ends of the cell. The course taken by 
the dendrites does not seem to be affected in the least by the 
tracts of fibres of the formatio reticularis, since it frequently 
takes them obliquely through a bundle of arcuate fibres. The 
dendrites are stout at their bases. They give off only a féw 
branches. The principal axis of the dendrite pursues a rather 
even course for a comparatively long distance, tapering gradu- 
ally to the end. Its surface bears but few gemmules. 
The axone may arise from the body of the cell, or, when 
the long axis of the cell is horizontal, from one of the den- 
drites. It takes a course directly for the opposite side of the 
oblongata, giving off collaterals near its point of origin and 
then remaining free from them. The function of such an axone 
is, doubtless, commissural. 
The internal structure of a commissural neurone from the 
right side of the oblongata is drawn in Fig. 45. The cytoplasm 
is always far less voluminous in proportion to the size of the 
nucleus than in the tract-neurones. The nucleus is almost in- 
variably oval in outline, with its major axis disposed the long 
way of the cell. The nucleolus is not conspicuous. The 
chromatic material is distributed in elongated, irregularly 
formed strands which lie throughout all parts of the nucleus. 
The several masses may be connected with each other, but only 
the faintest suggestion of it is indicated with the highest 
magnification. — 
The cytoplasm contains a considerable quantity of tigroid 
substance. A few broadly triangular tigroids are scattered 
round the nucleus, sometimes in the condition of nuclear caps. 
The greater part of the tigroid substance is to be found at the 
wide expanse where the cell-body merges into a dendrite. 
Here the form of tigroid is a greatly elongated triangle. This 
is replaced farther along the dendrite by spindle-shaped or linear 
masses. A small axone-hillock of finely granular matter lies 
just within the origin of the axone. The tigroid-bodies near it 
are smaller and more irregular than elsewhere. 
The commissural neurones, in contrast with the tract-neu- 
rones, always respond to the stains applied; this indicates a 
