CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
751 
In Acanthias the arrangement resembles very much that found in Rhina ; these 
cells begin suddenly some distance in front of the trochlear is and are restricted as a 
group in the centre of the roof of the optic ventricle ; anteriorly they terminate over 
the posterior edge of the posterior commissure, which in this species is some distance 
behind the anterior termination of the ventricle. 
These cells occur also in turtles, in which their arrangement resembles that 
described in the last two species ; in these animals they are grouped together in the 
projecting roof of the ventricle of the optic lobe in precisely the same manner as in 
Acanthias or Rhina. 
The function of these cells is not at first sight apparent; according to the theory 
that assigns motor functions to the larger cells and sensory functions to the smaller 
cells, these ought to be motor cells ; but their position is adverse to this view, the 
motor cells being usually, though not always, placed on the ventral side of the nervous 
centres. They may be probably looked upon as corresponding to the large cells in 
the nerve cell layer of the retina. These latter have been considered by some authors 
as refuting the above-mentioned theory, but, according to Professor M. Foster, ^ there 
is a residual motion in the eye after all the nerves have been severed, which he attri¬ 
butes to these cells in the retina. The cells in the optic lobe have probably some such 
function, and it would be interesting to know whether the retina in the Plagiostomata 
differ in any respect from the retina of those animals, e.g., Teleostei, which do not 
possess these cells in the optic lobe. 
Cerebellum. 
Two lobes may be distinguished in the cerebellum of Scyllium : one, anterior, pro¬ 
jecting over the surface of the optic lobe for quite half its length, and ending in front 
in a pointed extremity; and one behind, which in like manner extends over the fourth 
ventricle ; dorsally these coalesce and form one indistinguishable substance. Corre¬ 
sponding to the external shape, there is internally a ventricle which occupies the 
whole length of the lobe. Slightly in front of the central point there is a communica¬ 
tion below with the general cavities of the brain through the dorsal wall of the 
aqueduct of Sylvius. 
In structure the cerebellum (fig. 17) presents the four layers found in the brain of 
the Teleostei, that is to say, externally the molecular layer characterized by the 
striation directed perpendicularly to the external surface, then the layer of Purkinje 
cells or intermediate layer, internal to which occur the stratum of fibres, principally 
longitudinal, but in some places transverse, the fibrous layer, derived from the crura 
cerebelli ad medullam to a great extent ; finally the granular layer is the most internal 
and projects into the ventricle. 
The aggregate foldings of these four layers constitute the cerebellum, and in a 
* ‘ Textboolr of Physiology,’ p. 408. 
