504 THE NERVOUS SYSTEM. 



changes manifest themselves first in the body of the neurone itself and thence 

 spread along its processes. With the knowledge that protoplasmic bridges exist 

 long before the time His supposed the axon of his neuroblast to push its way 

 outward, it seems not unreasonable to suppose that it is the chemical modification 

 of these existing bridges which has been revealed in stained specimens, as it spreads 

 from the cell body outwards into its processes. 



It is now a well-recognised fact that soon after the neural tube becomes closed 

 the outlines of its constituent cells become blurred and then disappear, and a 

 continuous protoplasmic network or syncytium is formed. No one has ever been 

 able to detect the process of detachment of embryonic nerve-cells (neuroblasts) from 

 this syncytium ; and it is at least a possibility that the free anastomosis of the 

 protoplasmic processes of many of the cells is not destroyed in the way demanded 

 by the neurone doctrine. The known facts might be interpreted, at least as 

 reasonably, by supposing that when nerve currents begin to traverse the syncytium 

 (Fig. 444) structural modifications occur around the nuclei of the cells affected, and 

 gradually spread along their processes, so as to give the appearance (in sections 

 stained by special methods) of processes growing out from each neurone. 



Impulses brought from the skin by the sensory nerves, the nutrition of which is 

 controlled by the cells in the sensory ganglion (Fig. 443), are carried into the wall 

 of the neural tube, where they are received by processes of intercalated cells, which 

 in turn transmit their effects directly or indirectly to (a) motor nerve-cells (or 

 other kind of efferent nerve-cells), which stimulate a muscle, a viscus, or other 

 active tissue to perform some work, or (b) to intercalated cells, the axons of which 

 proceed to some other part of the nervous system, perhaps above or below the place 

 where the sensory nerve enters (Fig. 444, funicular cells). As the walls of the neural 

 tube increase in size the various neurones gradually become drawn apart, and the 

 protoplasmic links uniting them become stretched and extended to form processes 

 of varying length. 



It is right to explain that most writers give an explanation of the process of 

 development which is at variance with that just sketched. The neuroblast is 

 supposed to originate as a free-lying spherical cell, which is stimulated by some 

 unknown force, sometimes assumed to be of the nature of a chemical attraction 

 (chemotaxis), to protrude a process, which gradually elongates and pushes its way 

 through the tissues, perhaps to some particular patch of skin, muscle, gland, or 

 some other nerve-cell. The difficulty involved in such a conception is not only 

 that it is opposed to all that is known of the early stages in the evolution of the 

 nervous system, but also that it is difficult to conceive that every one of the 

 millions of nerve-cells, muscle-cells, visceral and cutaneous elements can each have 

 some specific attractive power which leads every individual nerve fibril to its 

 appropriate and predestined place in the body. 



The Efferent Nerves. The efferent cells of the neural tube are distinguished 

 by the fact that their axons leave the central nervous system and traverse the 

 mesoderm for a longer or shorter distance to end in relation to some muscle, gland, 

 or other tissue outside the nervous axis. At an early stage of development (Fig. 445) 

 such efferent fibres pass not only to muscles but also to viscera and other kinds of 

 tissues. In the course of the growth of the body these various structures supplied 

 by efferent fibres become removed progressively further and further from the central 

 nervous system ; and in this process a distinction can be detected in the behaviour 

 of the efferent fibres proceeding (a) to the striped or voluntary muscles, (c) and the 

 viscera and unstriped muscle, respectively. The efferent cells (a) which innervate 

 voluntary muscles retain their positions in the central nervous system, their axis- 

 cylinder processes (motor nerves) becoming elongated in proportion to the migration 

 of the muscle from its original situation. But thecells (c) innervating non-striped 

 muscles and viscera behave in a different manner. As the viscus or muscle 

 migrates (Fig. 445, B), the nerve-cell (c) follows it more or less closely, being as it 

 were dragged out of the wall of the neural tube by its axon into a peripheral 

 position, where it becomes a constituent element of one of the so-called sympathetic 

 or autonomic ganglia. As these sympathetic cells migrate from the central 

 nervous system, each of them appears to draw out with it the axon of an inter- 



