Houser, Zhe Neurones of a Selachian. 139 
comprising something like one-fourth the thickness of the en- 
tire thalamus. 
The neurones of this group are the largest of the thala- 
mus. The cell-body has a polygonal form, the several diam- 
eters not greatly unequal. The dendrites radiate freely in all 
directions, but they are not very long. The nucleus of the cell 
has an eccentric position, causing the cytoplasm to appear 
massed on the side from which the chief dendrites arise. The 
chromatic substance is disposed ina few thin strands having 
thickened nodes. The entire amount of chromatin is not great, 
and so the nucleus presents a lightly stained appearance. The 
tigroid substance is limited almost entirely to that part of the 
cytoplasm having the greatest mass. Some of the tigroids are 
altogether irregular in form and are relatively quite large. Fig. 
62 exhibits two neurones as they lie in place. 
The nucleus strati grisei is the terminal station for those 
axones of the tractus strio-thalamicus having their origin in 
the general striatum. These sweep into the nucleus in bundles, 
and their terminations are to be noted between the constituent 
neurones (Fig. 24, f. s. 2.). 
The neurones of the nucleus strati grisei are, primarily, a 
relay in the olfacto-motor chain. The tractus strio-thalamicus 
terminating here is one of the links of that chain, as we shall 
point out in detail under Section VIII. The axones from the 
cells of the nucleus strati grisei pass backward into the base of 
the midbrain as the tractus thalamo-tectalis, and then sweep 
upward into the tectum to lie in the stratum medullare profun- 
dum. Here they are associated with other sensory nerve-fibres, 
as already noted in Section VI, and the entire group becomes 
related to the remarkable motor conducting path provided by 
the cells of the roof-nucleus and the fibre of REISSNER. 
It is certainly not worth while, with the knowledge which 
we have at present, to attempt an extensive comparison of the 
- nucleus strati grisei with the specialized thalamic nuclei of 
higher vertebrates. It seems fairly safe, however, to regard 
the nucleus rotundus and the nucleus magnocellularis as descen- 
