VITl] THE NERVOUS SYSTEM 97 



instead of going directly to the organ to be supplied, terminate 

 at a nerve cell located in a ganglion. Such fibres are called 

 'pre-ganglionic' The nervous message is then transmitted 

 through another nerve fibre, which is therefore post-ganglionic 

 and innervates the organ or part concerned, in this case either a 

 secreting gland, or involuntary unstriated muscle. 



Examples of secreting glands which are thus innervated by 

 post-ganglionic fibres are the salivary glands, the gastric and 

 intestinal glands, the sweat glands and the lachrymal or tear 

 glands. Examples of involuntary muscle innervated in the 

 same kind of way are the muscles of the stomach and intestine, 

 those of the uterus and bladder, as well as the muscles of the 

 heart and blood vessels. All these muscles are uncontrolled by 

 the will, and act in a purely automatic way. 



The autonomic sj'stem which supplies involuntary muscle 

 and secreting glands may be divided into cranial, thoracic and 

 sacral nerves, according to the region of the central nervous 

 sj'stem from which thej^ take origin. The thoracic autonomic 

 system is frequently designated the sympathetic nervous s3-stem 

 owing to an idea which formerly prevailed regarding its nature. 

 It has already been shown in previous chapters that many organs 

 have two sources of autonomic nerve supply, the functions of 

 which are different or even antagonistic. Thus the heart is 

 excited to beat faster by stimulating the sympathetic supply, 

 and slowed do\\ii bj^ stimulating the vagus or cranial autonomic 

 fibres, the vagus nerves having been described as the 'reins of 

 the heart.' Similarly the arteries receive vaso-constrictor and 

 vaso-dilator fibres which arise from different regions of the 

 central nervous system, but the constrictor nerve fibres are 

 always thoracic autonomic (i.e. sympathetic) in origin, and it is 

 these fibres which are acted on so powerfully bj' adrenalin, 

 which is the active substance of the secretion of the suprarenal 

 body. 



The Central Nervous System. 



The body wall of the cranium and of the vertebral canal is 

 lined internally by a fibrous membrane called the diu'a mater. 

 The central nervous S3^stem throughout is lined by another 

 membrane termed the pia mater which is very vasciilar. In 

 between these membranes and loosely attached to them is a 



