Houser, The Neurones of a Selachian. 133 
tween the somatic motor neurones of the spinal cord and certain 
sensory impressions, visual and olfactory, at least. The classes 
of impressions noted are carried through the stratum medullare 
profundum to the terminal arborizations which we have described 
near the cells of the roof-nucleus. The neurones of the roof- 
nucleus transmit such impressions through their axones, REtss- 
NER’S fibre, directly to motor neurones at the several levels of 
the spinal cord. The apparatus of the roof-nucleus and ReEtss- 
NER’S fibre, regarded as a thing apart, is advantageous to Mus- 
telus because it is a path without relay, a short arc for motor 
reflexes between the eye and the olfactory organ, on the one 
hand, and the body musculature on the other. The giant size 
of the neurones of the roof-nucleus is doubtless the correlative 
of not only their long axones, but of their importance in the 
economy of the animal, as well. In ascending the scale of the 
vertebrate series, however, it will be found that this mechanism 
ever takes on a progressively less and less value, owing to the 
development of other means for attaining the same end. 
b. The Nucleus of the Oculomotorius.—The nucleus of the 
III nerve lies ventral to the aqueduct of SyLvius (Fig. 58, z. 
III). This collection of neurones is sharply marked off from 
all surrounding nervous elements by the large size of its cells 
and by the greater intensity of stain with methylen-blue. The 
group extends antero-posteriorly for some distance. 
The prevailing form of cell is oval, with two or three 
marked extensions of outline produced by the broadly triangular 
bases of the dendrites (Fig. 59). The axone is more slender 
by far than the dendrites ; it is directed away from the limitans 
interna, taking the course of a sweeping curve. 
The nucleus of the cell is evenly rounded in form, rather 
large as to proportionate size, and it has a central location. 
There is but a single nucleolus. The chromatin is distributed 
in a delicate network, the few interlacing strands visible being 
of great tenuity. 
The cytoplasm is remarkable for the large size of its masses 
of tigroid substance, these bodies being visible as distinct 
things even under low amplification. The form of a tigroid is 
