Houser, The Neurones of a Selachian. 87 
a. Tract-Neurones.—The ventral tract-neurones are scat- 
tered over the mid-ventral field of the oblongata on each side 
of the raphe ventral to the dorsal longitudinal bundles (Fig. a, 
t. 2.). They are to be readily distinguished from all other neu- 
rones of this region by their size, for they are really giants. 
Their size is exceeded only by the neurones of the midbrain 
roof-nucleus described in Section VI, Subsection 2. 
As to external form, the tract-neurones have a wide range; 
a representative individual is drawn in Fig. 2, ¢4. ~. There are 
from three to five dendrites, and their disposition controls the 
shape of the cell-body to avery high degree. The dendrites 
may be given off at opposite extremities of the cell, in which 
case the outline of the cell-body is a much elongated one. The 
form is rounded or stellate when the dendrites are spaced at 
equal intervals. A dendrite is always a massive process, very 
wide at its base, tapering quite gradually, and reaching far out 
into the surrounding nervous matter. It gives origin to but 
few branches. 
The axone always arises from the body of the cell. It runs 
for a short distance in the transverse plane and then turns into 
a longitudinal bundle of fibres on the same side of the oblon- 
gata, or even on the opposite side. 
The internal organization of a tract neurone is shown in 
Fig. 44. The cytoplasm is voluminous in quantity, investing 
the nucleus with a thick layer on every side. The outline of 
the nucleus is regular, and in form may be circular or oval. 
There are always a remarkably small number of coarse chrom- 
atin granules, the chromatic material being distributed in the 
form of a delicate reticulum. Subsidiary nucleoli are rarely 
present. 
The cytoplasm is, as already noted, great as to actual 
quantity. Some neurones when stained by the method of Nissi 
absorb the methylen-blue equally throughout all parts of the 
cytoplasm, and hence they appear almost homogeneous ; the 
significance of this fact is noted below. Other neurones in the 
same section exhibit a large quantity of tigroid substance ; the 
neurone represented in Fig. 44 is of this type. In the vicinity 
