96 ZOOLOGY SECT. 



The spinal cord (Fig. 786) is a thick-walled cylinder, con- 

 tinuous in front with the brain. It is traversed from end 

 to end by a narrow central canal (3), lined by ciliated epithelium 

 derived from the superficial layer of in-turned ectoderm cells. 

 The dorsal surface of the cord is marked by a deep, narrow, longi- 

 tudinal cleft, the dorsal fissure (2), the ventral surface is similarly 

 scored by a groove, the ventral fissure (1) ; owing to the presence of 

 these fissures a transverse section presents two almost semi- 

 circular halves with their straight edges applied to one another 

 and joined in the middle by a narrow bridge (4, 5) in which the 

 central canal lies. 



The cord is made up of two kinds of tissue. Surrounding the 

 central canal and having a somewhat butterfly-shaped transverse 

 section, is the grey matter (a. e) consisting of delicate, inter-twined, 

 non-medullated nerve-fibres, amongst which are numerous nerve- 

 cells. The superficial portion is composed of medullated nerve-fibres 

 running longitudinally, and is called the white matter (6, 7, 8). In 

 both grey and white matter the nervous elements are supported 

 by a non-nervous tissue called neuroglia, formed of branched cells. 



From the cord the spinal nerves are given off. They arise in 

 pairs from the sides of the cord, and agree in number with the 

 myomeres. Each nerve arises from the cord by two roots, a 

 dorsal and a ventral. The dorsal root (Fig. 788, d. r.) is distin- 

 guished by the presence of a ganglion (gn. d.r.) containing nerve-cells, 

 and its fibres are almost exclusively afferent, conveying impulses 

 from the various' parts and organs of the body to the central 

 nervous system ; the ventral root (v. r.) is not ganglionated, and 

 its fibres are efferent, conveying impulses from the neuron outwards. 

 Each root arises from one of the horns of the grey matter, and 

 the two mingle to form the trunk (sp. 1-3) of the nerve, which 

 emerges from the spinal canal usually between the arches of 

 adjacent vertebrae. Soon after its emergence it divides into two 

 chief divisions, dorsal (d.) and ventral (sp. 1, &c.). The spinal 

 nerves supply the muscles and skin of the trunk and limbs, and 

 are therefore spoken of as somatic nerves. Frequently groups of 

 nerves unite with one another to form more or less complex net- 

 works called plexuses. 



Closely associated with the spinal are the sympathetic nerves 

 (Fig. 788, sym.). They take the form of paired longitudinal cords 

 with ganglia (sym. gn.) at intervals, lying one on each side of the 

 aorta in the dorsal wall of the ccelome. They contain both 

 afferent and efferent fibres, the afferent derived from the dorsal, 

 the efferent from the ventral roots of the spinal nerves, and both 

 traceable, through those roots, into the grey matter of the cord. 

 The sympathetic nerves supply the enteric canal and its glands, 

 the heart, blood-vessels, &c., and are therefore denominated 

 splanchnic nerves. 



