1336 A MANUAL OF ANATOMY 



floor-plate and roof-plate. Each lateral wall is divided into two plates — 

 ventral and dorsal — by a longitudinal groove, called the sulcus limitans. 

 The ventral plate, which is motor, is spoken of as the basal lamina, and the 

 dorsal plate, which is sensory, is referred to as the alar lamina. The basal 

 lamina is thick, and its mantle zone contains many cells from which the 

 anterior cornu of grey matter is developed. The axons of these cells pass 

 outwards through the marginal zone and form the anterior roots of the spinal 

 nerves. The fibres of the posterior nerve-roots enter from the spinal gangUa 

 at the sulcus limitans, and in the marginal zone of the alar lamina they form 

 by their bifurcations longitudinal strands, which constitute the posterior 

 column of the cord. 



Metamorphoses of the Primitive Spinal Cord. — ^The primitive spinal cord 

 consists of (i) two lateral walls, (2) a ventral or floor-plate, and (3) a dorsal 

 or roof -plate. 



Changes in Lateral Walls. — Each latereil wall, as stated, is subdivided into 

 a ventral or basal lamina and a dorsal or alar lamina by the sulcus limitans. 

 The ventral or basal lamina on either side gives rise to (i) the anterior cornu 

 of grey matter, (2) the anterior or motor-nerve roots, and (3) the longitudinal 

 funiculi of nerve-fibres, which form the anterior and lateral ground-bundles. 

 The dorsal or alar lamina on either side gives rise to (i) the posterior cornu 

 of grey matter, including the substantia gelatinosa of Rolando, and (2) the 

 longitudinal funiculi of nerve-fibres which form the posterior column of the 

 cord (Goll's column and Burdach's column), which receive most of the fibres 

 of the entering posterior nerve-roots. 



Lumen of Neural Tube. — This constitutes the central canal of the spinal 

 cord. At first it is a dorso-ventrally elongated slit, widest in the dorsal 

 portion, and subsequently in the ventral region. At a later period the dorsal 

 portion of the lumen becomes closed, but the ventral portion, now somewhat 

 round, persists as the central canal of the cord. The closure of the dorsal 

 portion of the lumen occurs during the development dorsalwards of the 

 posterior comua of grey matter and posterior columns of the cord. The 

 lateral walls of the dorsal portion, becoming approximated, fuse, the dorsal or 

 roof-plate disappearing. Along the line of fusion neuroglia-fibres extend 

 from the dorsal wall of the central canal to the dorsal surface of the cord at 

 the median hne. This thin layer of neuroglia-fibres constitutes the posterior 

 median septum. As the posterior columns of the cord undergo development 

 the septum elongates dorsalwards, the dorsal or roof -plate disappearing. 



Anterior Median Fissure. — This fissure is the result of the growth ventral- 

 wards of the anterior column of each side. During this ventral growth the 

 ventral or floor-plate becomes depressed, and consequently a distinct cleft 

 is produced, ventral to the depressed floor-plate, and intervening bet\veen 

 the protuberant anterior columns. This cleft forms the anterior median 

 Assure of the cord, which is occupied by an inflection of the pia mater. The 

 floor-plate now lies at the bottom of the fissure, and that plate forms the 

 anterior white commissure of the cord. It consists of decussating nerve- 

 fibres, derived from the neuroblasts of the mantle layer of either side. 



For the development of the spinal ganglia and the sensory and motor 

 roots of the spinal nerves, see Neural Crests under Development of the Periph<?|^ 

 eral Nervous System. 



Membranes of Spinal Cord. — ^The membranes — namely, pia mater, arach- 

 noid, and dura mater (theca) — are developed from the mesoderm which! 

 invests the neural tube. 



Growth of Spinal Cord. — The cord originally occupies the entire length of j 

 the spinal canal of the vertebral column. The vertebral column, however, grows J 

 more rapidly than the cord, so that at the period of birth the cord does not] 

 extend lower than the level of the third or fourth lumbar vertebra. Sub- 

 sequently its lower limit is the intervertebral disc between the bodies of thej 

 first and second lumbar vertebrae. This produces a change in the course of] 

 the lumbar, sacral, and coccygeal nerves. In order to reach the level of the] 

 intervertebral foramina through which they emerge from the spinal canall 

 they descend almost vertically, and constitute the bundles of nerves known] 

 as the Cauda equina, 



