MAMMALIA 299 



ventral fissures, which nearly reach to the central canal. The 

 grey matter is central, and in section appears as dorsal and 

 ventral horns on each side. This contains the nerve cells, and 

 the axons forming the white superficial matter are arranged 

 in longitudinal tracts. The spinal nerves arise in dorsal and 

 ventral roots, which are united to form a common nerve. The 

 dorsal root bears a ganglion in each case, is sensory, and is 

 connected to the dorsal horn ; the ventral root does not bear 

 a ganglion, is motor, and arises from the ventral horn. The 

 former is afferent, and the latter efferent. The spinal nerves 

 innervate the skin and wall of the body, and through the 

 sympathetic system are also related by sensory and motor 

 fibres to the viscera. Thus simple nervous impulses affecting 

 the body alone may be conducted without reference to the 

 brain that is, without consciousness. 



The first spinal nerve is situated between the skull and the 

 first vertebra, and the succeeding nerves come out in front of 

 the corresponding vertebrae. With the exception that there 

 are eight cervical spinal nerves, the numbers agree with the 

 numbers of the vertebrae. The brachial plexus is formed by 

 the association of the spinal nerves from the fifth cervical to 

 the first thoracic, and it supplies the shoulder and the fore- 

 limb. The fourth cervical, with branches from the fifth and 

 often the sixth, sends a phrenic branch to the diaphragm. It 

 runs along the anterior and posterior venae cavae to reach 

 its destination. The fifth, sixth, and seventh lumbar nerves 

 and the sacral nerves combine to form the lumbosacral plexus, 

 yielding the crural, obturator, and the sciatic nerves to the 

 gluteal region and the leg. They are succeeded by the caudal 

 nerves, which are carried backwards from the end of the cord 

 to pass out in succession between the caudal vertebrae. 



The sympathetic nervous system consists of a chain of 

 ganglia developed from the spinal ganglia and forming two 

 longitudinal strands which extend from the cranial nerves V, 

 IX, and X to the hinder part of the body. In the neck are 

 anterior and posterior cervical ganglia, and thereafter ganglia 

 corresponding to each spinal nerve, with which each ganglion 

 is connected by a ramus communicans. The ganglia consist 

 of multipolar cells and distribute impulses to the heart, vessels, 



