Houser, The Neurones of a Selachian. 125 
two lateral halves are united by a strong commissure just dorsal 
to the aqueduct of Syivius. Fig. 20, s. m. ~., represents these 
features in a transverse section. 
Traced laterally, the fibres of the stratum medullare pro- 
fundum are found to pass into the base of the midbrain in two 
somewhat clearly marked divisions. The outer division takes 
a large number to the lateral surface (Fig. 20, ¢. 4); while a 
markedly smaller group passes downward nearer the median 
line for a ventral decussation (Fig. 20, z. 7). The inner division 
is composed of those fibres lying nearer to the central gray 
matter from the outset. Some of these have but a short course 
downward and outward, but the great mass of fibres continues 
near the median line as a series of intercrossing bundles which 
are destined to decussate ventral to the aqueduct of SyLvius 
(Mig. 20, ¢.°., 2,72.). 
The fibre-system composing the stratum medullare profun- 
dum must be indeed important in the economy of the brain. 
Into this system we have traced fibres from the cord and ob- 
longata, fibres from the optic nerve, and from ‘the neurones of 
the tectum mesencephali itself. We will subsequently have 
occasion to point out that a great tract sweeps into it from the 
interbrain as a relay in the olfactory apparatus. Fibres are also 
present here from certain of the cranial nerves. All of these 
fibre-systems from diverse sources are to become related to the 
remarkable mechanism of REISSNER’s fibre. It is certainly 
evident that there are here every means for intercommunication 
between different parts of the nervous system, a switch-board, 
so to speak, of extraordinary possibilities; but to this subject 
we shall return further on. 
b. The Superficial Neurones.—The outermost layer of the 
tectum mesencephali is characterized by the predominance of 
nerve-fibres and by the feeble development of its neurones. 
Receiving as it does the fibres of the optic nerve for their first 
expansion, we should hardly expect a high degree of nervous 
activity here. There are a few scattering nerve-cells present, 
however, (Fig. 21, s. .). These are quite minute in size, 
lenticular in form, and are disposed with their long axes tan- 
