240 THE ENGINES OF THE HUMAN BODY 



laugh or cry ; they are the seat of all emotional disturb- 

 ances. Likewise in the brain stem itself, in the medulla, 

 and in the spinal cord the nerve operatives are grouped 

 in stations or centres occupying the deeper or inward 

 parts. In the spinal cord, which is imperfectly divided 

 into right and left columns, we find the white matter, 

 made of the cables uniting the operatives of one station 

 with those in a distant station, set on the surface, while 

 the grey matter forms a core in each side column (fig. 

 45, C). A bridge-work crosses within the spinal cord and 

 unites the grey cores and the enclosing white cables 

 of the opposite sides. The spinal cord, with its central 

 cores of grey matter and outer covering of white matter, 

 represents the original manner in which the central nerve 

 machine was constructed. It was only when Nature 

 came to elaborate the cortical areas of the mammalian 

 brain that she reversed her method and spread the 

 operative units on the surface and placed the communi- 

 cating cable in the centre. The adoption of that simple 

 modification in construction gave her, as Prof. Elliot 

 Smith has demonstrated, an unlimited field for expanding 

 the cortical areas — a freedom which made the human 

 brain possible. In birds and reptiles Nature pursued her 

 old method, packing their cortical substance in the interior 

 of the hemispheres, with the result that their further 

 growth soon became impossible for lack of space. 



We have made a rapid survey of the great central 

 nerve machine. We must now glance at the cables of 

 living wires which place it in communication with every 

 part of the human body. From the spinal cord there 

 emerge thirty-one pairs of nerves (fig. 45) destined for 

 the right and left halves of the body and the limbs 

 attached to these halves. Each nerve issues from the 

 cord in two divisions or roots. On the hinder or 

 posterior root (fig. 45, C) is a swelling or ganglion, packed 

 full of the kind of operative we may name message- 

 transmitters. Each one of them is set upon — forms an 

 intrinsic part of — the nerve fibre or wire through which 

 its messages are carried. The spinal nerve-trunks issue 



