Houser, The Neurones of a Selachian. 83 
The effort to compare the cranial with the spinal nerves on the 
simple basis of ‘‘sensory’”’ and ‘‘motor’’ could not avoid leading 
to many dogmatic positions concerning the real character and 
ultimate distribution of many fibres. An attempt to solve such 
intricate problems through so mechanical a method could 
hardly be otherwise than faulty, particularly when applied to 
the specialized conditions of the mammalian nerves. A thor- 
ough study of the less modified cranial nerves of the Ichthy- 
opsida is a necessary preparation for sound morphological work 
in the higher field. 
The views of cranial nerves held by the neurologists of 
to-day were founded less than a decade since, but the germ of 
the central idea is traceable to a somewhat earlier date. Gas- 
KELL in a series of publications (86, ’88, 89) was making the 
attempt to solve the metamerism of the head and the origin of 
the vertebrate nervous system through a study of the nerves. 
He took occasion to show that a spinal nerve not only embraces 
the sensory and motor fibres of Berr, but that its structure, 
distribution, and function, as well as the arrangement of its 
central nuclei, lead to the divisibility of the nerve into two 
parts. One part is somatic, innervating the external surface of 
the body, and the muscles derived from the muscle-plates. The 
other division is splanchnic, supplying the internal organs and 
surfaces, and those muscles which GASKELL characterizes as 
“derived from the lateral plates of the mesoblast.’”’ GASKELL 
attempted to show, further, that the cranial nerves arise from 
centres homologous with the spinal centres, likewise divisible 
into somatic and splanchnic groups. t 
The impetus given by GASKELL has led, ultimately, to the 
modern conception that a spinal nerve embraces neurones de- 
rived from four distinct sources, with as many different distribu- 
tions. There are, then, to be distinguished in a spinal nerve: 
(2) Somatic motor fibres. These neurones have their cell-bodies 
situated in the ventral cornu of the cord, their axones emerge 
through the ventral root, and they are distributed to the body 
musculature. (4) Somatic sensory neurones, the cell-bodies of 
which comprise the dorsal ganglion, and their fibres connect 
