THE MASTER CONTRIVANCE 237 



substance. The capillary fields are freely fed by large 

 arteries and drained -by veins into special channels — the 

 venous sinuses. The great machine itself and the pipes 

 laid on to provide it with a constant supply of fuel and 

 oxygen are packed away in the neatest sailor-like fashion. 

 The cranial chamber is divided into a large upper and a 

 small lower storey by a horizontal membranous partition, 

 in which there is an open doorway giving passage to the 

 great brain stem (fig. 45, A). In the upper compartment 

 are lodged the two great cerebral hemispheres, right and 

 left, linked together by a great bridgework of nerve fibres 

 — the corpus callosum. The right hemisphere, we shall 

 find, presides over the left half of the body, and the left 

 one over the right half. It is the left hemisphere, for 

 example, which regulates the movements of the right 

 hand. Then in the smaller and lower storey is housed 

 the cerebellum and also the continuation of the brain 

 stem which issues from the cerebral hemispheres in the 

 upper compartment. The part of the stem which lies in 

 the lower storey, and to which the cerebellum is linked 

 by great nerve cables, is the medulla, or, to give its name 

 in full, the medulla oblongata. In former chapters we 

 have often had occasion to refer to the vital controlling 

 centres lodged within the medulla, centres which regulate 

 the pumping of blood by the heart, ventilation of the 

 lungs, and management of the alimentary laboratories. 

 Through a wide opening — the foramen magnum — in the 

 base of the skull the cranial chamber is prolonged into 

 the spinal chamber, an elongated corridor in which is 

 lodged a continuation of the brain stem — the spinal cord. 

 Thus the great central nerve machine, which we are now 

 examining, extends from the forehead to the lumbar 

 region of the spine — the small of the back. 



In the great alimentary laboratories we found that the 

 living microscopic units— each a minute fuel factory — 

 were grouped so as to form a lining for the chambers. 

 In the cerebral hemispheres or cerebrum, and the same 

 is true of the cerebellum, we find Nature has adopted 

 an opposite plan. The microscopic operatives which 



