still very considerable doubt. With disease or willful destruction of
the cerebral tissue the personal initiative is affected-- the animal
becomes more distinctly a mechanism; the cerebellum is probably
concerned in the coordination of muscular movements; and the
medulla is a centre for the higher and more complicated respiratory
reflexes, yawning, coughing, and so on. The great majority of reflex
actions centre, however, in the spinal cord, and do not affect the
brain.
Section 131. A cross section of the spinal cord is shown in Figure 6,
Sheet 8. It is a cylinder, almost bisected by a dorsal (d.f.) and a
ventral (v.f.) fissure. Through its centre runs a central canal (c.c.),
continuous with the brain ventricles, and lined by ciliated epithelium.
The spinal cord consists of an outer portion, mainly of nervous fibres,
the white matter, and of inner, ganglionated, and more highly
vascular grey matter. (In the cerebrum the grey matter is external, and
the white internal.) The cord, like the brain, is surrounded by a
vascular fibrous investment, and protected from concussion by a
serous fluid. The nerves which emerge from the vertebral column
between the vertebrae, arise, unlike the cranial nerves, by two roots.
The dorsal of these, the sensory root (d.n.), has a swelling upon it, the
dorsal ganglion, and-- by experiments upon living animals-- has been
shown to contain only afferent fibres; the ventral, the motor
root, is without a ganglion, and entirely or mainly motor. The two unite
outside the cord, and thereafter the spinal nerves are both sensory
and motor.