622 NERVOUS SYSTEM 
I. The Spinal Cord or “Marrow” is the continuation of the 
medulla oblongata, extending throughout the vertebral canal 
to the tail, being swollen at the level of the shoulders by an 
accumulation of ganglionic cells to serve the Brachial Nerve 
Plexus governing the fore-limbs, and again in the lumbar region 
where the nerves of the hind-limbs have their origin. On the 
dorsal side of this lumbar swelling, there is a lozenge-shaped slit, 
the sacro-rhomboidal sinus, filled with a colourless gelatinous 
substance, behind which the cord lessens gradually, ending as 
a thin thread in the last free caudal vertebra. The whole of 
this System is: encased in a strong, fibrous sheath of connective 
tissue, the dura mater, the outer layer of which is closely attached 
to and forms the lining of the central vertebral canal, while its 
inner layer forms a looser and more meshy tissue. A much thinner 
membrane, the pia mater, is immediately attached to the surface of 
this System, penetrating its various furrows or sulci, and containing 
blood-vessels which nourish the nervous matter. Between the dura 
and the pia mater, but partly separated from each by lymphatic 
spaces, lies the Arachnoid Membrane. 
The composition of the Spinal Cord is best studied in transverse 
section :—in the midst is the central canal, on the medio-ventral 
and medio-dorsal lines are the anterior and posterior sulci, forming 
more or less deep vertical slits which thus divide the cord into a 
right and left side. The central portion of the cord, distinctly 
grey in colour, being composed of grey nerve-fibres, without axial 
cylinders, and interspersed with numerous ganglionic nerve-cells, 
arranged in the form of a saltire or St. Andrew’s cross, of which the 
ventral pair of limbs contain the ganglia and send out the motory 
roots of the spinal nerves, while the dorsal pair contain the ganglia 
and send out the sensory bundles of nerve-fibres. This grey matter 
is surrounded by a thick mantle of white nerve-fibres, most of them 
conducting threads composed of an axial cylinder with a sheath, 
and running longitudinally parallel to each other, though at the 
so-called commissures a crossing from one side to the other occurs 
with many of both white and grey fibres. 
II. The Spinal Nerves arise from the medulla by a number of 
rootlets, which leave its surface in the form of a dorsal and a 
ventral root—the former (with a small swelling, the spinal ganglion, 
at its base) containing the sensory, the latter the motory fibres ; 
but all the fibres of each issue as a single bundle either between 
two vertebre or pass through a hole at the anterior end of a 
vertebra. The first spinal or cervical nerve issues between the 
occiput and the atlas, and each that follows from the anterior end 
of its vertebra—all on leaving the vertebral column separating into 
three branches—a dorsal, ventral and visceral. The first two 
contain sensory and motory nerves mixed ; but the dorsal branches 
