680 THE NEKVOUS SYSTEM. 



and are secondarily connected with the dorso-lateral aspect of the medullary tube as the 

 fibres of the posterior root ; and (2) a peripheral series, which constitute "the posterior 

 root-fibres of the spinal nerve and join the anterior root, to form the spinal nerve proper. 

 It is only after the appearance of these nerve-fibres that the ganglionic crest becomes 

 notched along its peripheral border, and it is gradually divided up to form the individual 

 segmental spinal ganglia. 



The anterior root of a spinal nerve arises in quite a different way, from cells (neuro- 

 blasts) in the substance of the medullary tube. In the account of the development of the 

 spinal medulla it has been shown how the cellular constituents of the medullary tube 

 are converted into two classes of cells : (1) spongioblasts, which produce the matrix 

 (neuroglia) of the spinal medulla; and (2) neuroblasts, which produce the nerve-cells 

 of the gray matter of the spinal medulla. The neuroblasts give rise to the axis- 

 cylinder processes or axons, which, penetrating the spongy tissue of the medullary tube 

 and the outer limiting membrane, find their way into the mesodermic tissue on the 

 ventro-lateral surface of the tube. Fibrillar from their earliest origin and derived from 

 nerve-cells which remain within the medullary tube, the axons of the anterior root become 

 surrounded by mesodermic cells immediately on their emergence, which give rise to the 

 sheaths of the nerve. The anterior root is a little later in its date of appearance than 

 the posterior root. It begins to be evident at the twenty-fourth day and is completely 

 formed by the twenty-eighth day. 



II. Formation of the Spinal Nerve. The fibres of the posterior root ganglion and 

 the anterior root grow by extension from the cells with which they are respectively con- 

 nected, and meet in the space between the myotome and the side of the medullary tube 

 to form the spinal nerve. In the adult there is a fundamental division of the spinal 

 nerve into posterior and anterior rami. In the process of development this separation is 

 even more obvious. As the fibres of the posterior and anterior roots approximate, they 

 separate at the same time each into two unequal portions : the smaller parts of the 

 two roots unite together to form the posterior ramus, and the larger parts unite to form 

 the anterior ramus of the spinal nerve. 



The posterior ramus, curving laterally and dorsally, passes through the myotome 

 and is connected with it. In the substance of the myotome it separates into branches 

 as it proceeds towards the dorsal wall of the embryo. At a later stage, the branches 

 are definitely arranged into a lateral and a medial series. 



The anterior ramus grows gradually in a ventral direction to reach the somato- 

 splanchnopleuric angle, under cover of the growing myotome. It spreads out at its 

 distal end and eventually separates into two portions : a smaller, splanchnic, or visceral ; 

 and a larger, somatic, or parietal portion. (1) The smaller, splanchnic, or visceral portion 

 grows inwards, dorsal to the Wolman ridge, to be connected through the sympathetic trunk 

 with the innervation of organs in the splanchnic area. This branch of the spinal nerve 

 becomes the white ramus communicans of the sympathetic. It is not present in the case 

 of all the spinal nerves, but only in relation to the thoracic and upper lumbar and the 

 third and second or fourth sacral nerves. It will be referred to again in connexion with 

 the sympathetic system. (2) The larger, somatic, or parietal portion becomes the 

 main part of the anterior ramus of the nerve. It continues the original ventral 

 course of the nerve, and, reaching the body wall, subdivides into two terminal branches 

 a lateral branch, which grows laterally and downwards and reaches the lateral aspect 

 of the trunk, after piercing the myotome ; and a ventral or anterior branch, which grows 

 onwards in the body wall to reach the ventral axis. This arrangement is met with in 

 the trunk between the limbs and in the neck. 



III. Formation of Limb-plexuses. The method of growth of the spinal nerves, 

 just described, is modified in the regions where the limbs are developed. In relation to 

 the limbs, which exist in the form of buds of undifferentiated cellular mesoblast before 

 the spinal nerves have any connexion with them, the development of the anterior ramus 

 of the nerve proceeds exactly in the way described, up to the point of formation of 

 somatic and splanchnic branches. The somatic branches then stream out into the 

 limb bud, passing into it below the ends of the myotomes and spreading out into a 

 bundle of fibres at the basal attachment of the limb. Later, the nerves separate, each 

 into a pair of definite trunks, which are named posterior or dorsal and anterior or ventral, 

 and which, dividing round a central core of mesoderm, proceed to the dorsal and ventral 

 surfaces respectively of the limb bud. While this process is going on, a secondary wiion 

 takes place between parts of adjacent dorsal and ventral trunks. Dorsal trunks unite 

 with dorsal trunks, ventral trunks unite with ventral trunks, to form the nerves distri- 

 buted ultimately to the surfaces and periphery of the limb. These dorsal and ventral 



