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 vertebrse. 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 vertebrae 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 



