92 ZOOLOGY SECT. 



The nervous system attains a complexity, both anatomical and 

 histological, unknown in the rest of the animal kingdom. It 

 arises, as in other Chordata, from a dorsal medullary groove the 

 edges of which unite and enclose a tube. From the ectoderm 

 lining the tube the whole central nervous system, or neurones formed, 

 its lumen forms the neuroccde or characteristic axial cavity of the 

 neuron. So far the agreement with the lower Chordata is com- 

 plete, but a fundamental advance is seen in the fact that at an 

 early period before the closure of the medullary groove the 

 anterior end of the neuron undergoes a marked dilatation and 

 forms the rudiment of the brain, the rest becomimg the spinal 

 cord. Moreover, as growth goes on a space appears in the meso- 

 derm immediately surrounding the nervous system, and forms the 

 neural or cerebro-spinal cavity already referred to (Fig. 715, cs. c.), 

 so that the neuron, instead of being solidly imbedded in mesoderm, 

 lies in a well-marked and often spacious tube enclosed by the 

 neural arches of the vertebrae, and in front by the cranium 

 (Fig. 715, B-D). 



The spinal cord (Fig. 732) is a thick-walled cylinder, continuous 

 in front with the brain. It is transversed 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 sub- 

 stance of the cord itself being formed from the deeper layers. 

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

 longitudinal groove, the dorsal fissure (#), the ventral surface is 

 similarly scored by a 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, 6'). 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. 734, d. r.) is dis- 

 tinguished by the presence of a ganglion (gn. d.r.) containing 

 nerve-cells, and its fibres are usually wholly 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 



