ii8 



THE BIOLOGY OF BIRDS 



afferent) are made up of portions of sensory neurones, 

 carrying messages inwards (as the diagram suggests). The 

 ventral roots (otherwise called anterior or efferent) are 

 made up of portions of motor neurones, carrying commands 

 outwards. Outside the spinal cord, on each dorsal root, 

 there is a spinal ganglion (sp.g.), and this consists of 

 sensory neurones. From each of these a fibre runs into 



D.R sp.(^ 



Fig. 28. — Diagram of the elements involved in a reflex action, 

 sp.c, the spinal cord ; as.n., an associative neurone ; m.n., a motor 

 neurone ; v.r., a ventral root of a spinal nerve ; m.f., a motor nerve- 

 fibre ending in a motor plate (m.pl.) on a muscle (mu.) ; s.E., ending of a 

 sensory nerve-fibre, c.^. on the skin; s.f., a sensory nerve fibre; sp.g., 

 a spinal ganglion on a dorsal root (d.r.) of a spinal nerve ; s.N.,a sensory 

 neurone in the spinal ganglion ; br., branches of a sensory nerve-fibre 

 within the spinal cord. The arrows indicate the direction of the 

 message or command. Branches of a sensory fibre come into close 

 contact with branches of an associative neurone ; and similarly there is 

 close contact or synapsis between other branches of the associative cell 

 and those of a motor neurone. 



the spinal cord and another runs out to the periphery, 

 terminating in a sensitive nerve-ending on the skin or in a 

 sense-organ. The ventral roots are made up of nerve- 

 fibres, each of which grows out from a motor neurone in 

 the spinal cord and is continued to a nerve-ending on a 

 muscle or, it may be, to a gland. Finally, as middlemen 

 between the internal branches of the sensory nerve-fibres 

 and the internal branches of the motor neurones, there are 

 associative neurones (otherwise called communicating or 

 internuncial). Thus we have, as in figure 28 : — 



