THE BIOLOGY OF HEALTH 135 



into unity ; (2) the spinal cord — a complicated tract 

 of pathways apparently innumerable and of subordinate 

 centres ; (3) the nerves which issue from the brain and 

 the spinal cord, some consisting of fibres which carry 

 messages only inwards (sensory or afferent), some con- 

 sisting of fibres which carry messages only outwards 

 (motor or efferent), and some consisting of fibres of 

 both kinds ; (4) the sympathetic system — a series of 

 small nerve-centres, connected with the brain and 

 spinal cord, which have to do with the control of blood- 

 vessels, food-canal, heart, and so on. The sensory nerves 

 carry messages from superficial sense-organs or sense- 

 cells, and from sensory nerve-endings on internal organs. 

 The motor or efferent nerves carry commands to muscles 

 stimulating them to contract, and to glands stimulating 

 them to secrete. There are also efferent nerves that 

 command inaction, inhibiting rather than stimulating. 

 (B) The nervous system consists of millions of micro- 

 scopically minute elements — nucleated cell-bodies — 

 which receive, shunt, store, and combine messages 

 received from the outside world and from the recesses 

 of the body, and which also send out orders to the muscles 

 and glands and other parts. The messages that come 

 in and go out are carried, in some way that we do not 

 understand, by nerve-fibres, which are very delicate 

 outgrowths of the cell-bodies. The cell-body and its 

 fibres taken together form a nerve-cell or nerve-unit, 

 or neuron. The cell-bodies (often called the ganglion 

 cells) are like the telephones of a telephonic system, and a 

 certain set of them form the ' centrgfl.' The nerve-fibres 



