* A, G8 e OY 
620 THE NERVOUS SYSTEM. 
membrane, find their way into the mesoblastic tissue on the ventro-lateral surface of the 
tube. Fibrous from their earliest origin and derived from nerve-cells which remain within 
the medullary tube, the axons of which the ventral root is composed become surrounded 
by mesoblastie cells immediately on their emergence, which give rise to the sheaths of the 
nerve. ‘The ventral root is a little later in its date of appearance than the dorsal root. 
It begins to be evident at the twenty-fourth day and is completely formed by the twenty- 
eighth day. 
Il. Formation of the Spinal Nerve.—The fibres of the dorsal root ganglion and 
the ventral 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. It has been already shown that in the adult there is a funda- | 
mental division of the spinal nerve into posterior and anterior primary divisions. In the 
process of development this separation is even more obvious. As the fibres of the dorsal 
and ventral 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, and the 
larger parts unite to form the anterior primary division of the spinal nerve. 
The posterior primary division, curving outwards 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 an outer and an inner set. 
The anterior primary division 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 pé arietal portion. (1) The smaller, splanchnic, or visceral portion 
erows iInw vards, dorsal to the Wolffian ridge, to be connected with the sympathetic cord 
and 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 (cervical, lower lumbar, and upper sacral). It will be referred to 
again in connection with the sympathetic system. (2) The larger, somatic, or parietal 
portion becomes the main part of the anterior primary division of the nerve. It 
continues the original ventral course of the nerve, and, reaching the body wall, sub- 
divides into two terminal branches—a lateral branch, which grows outwards and down- 
wards 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 cellular undifferentiated mesoblast before 
the spinal nerves have any connection with them, the development 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 on, the nerves separate each into a pair of definite trunks, 
which are named dorsal and ventral, and which, dividing round a central core of meso- 
blast, proceed to the dorsal and ventral surfaces respectively of the limb bud. While this 
process is going on, a secondary union takes place between (parts of) adjacent dorsal and 
ventral ‘pavialse. Dorsal trunks unite with dorsal trunks, ventral trunks unite with ventral 
trunks, to form the nerves distributed ultimately to the surfaces and periphery of the 
limb. ‘These dorsal and ventral trunks are homologous with the lateral and ventral 
branches of the somatic nerves in other regions. 
MORPHOLOGY OF THE LIMB-PLEXUSES. 
The arrangement of the limb nerves is rendered complex and the significance of the plexuses 
is obscured by the changes through which, coincidently, the nerves on the one hand and the 
parts supplied by them on the other hand have passed in the course of development. 
Nature of the Limbs.—As already described, the mammalian limbs arise as flattened buds 
from the extremities of the Wolffian ridge. Each bud possesses a preaxial and a postaxial border, 
and a dorsal and a ventral surface, continuous with the dorsal and ventral aspects of the trunk 
and homologous with its lateral and ventral surfaces. Each bud consists at first of a mass of 
undifferentiated, unsegmented mesoblast, covered by epithelium. Around the central core of 
mesoblast which produces the skeletal axis, the vessels and muscles of the limb are formed 
in situ, the muscles as double dorsal and ventral strata, beneath the corresponding surfaces of 
the bud. 
Each limb bud is connected to the lateral and ventral aspects of the trunk, and is associated 
with a number of body segments, varying in the two extremities and in ‘different animals. 
