100 THE MECHANISM OF LIFE 



All sensory stimuli from the skin, tlie muscles, joints, and viscera 

 pass into the spinal cord via the dorsal (or sensory) roots of 

 the spinal nerves, and either go directly up to the brain or they 

 enter the grey matter and undergo " relays "* there — that is, 

 they are shunted on to new paths. Some of them pass out from 

 the cord via relays and the motor roots, when reflex actions occur 

 (see later), but the others travel up to the brain along tracts in 

 the white matter. The great ascending tract in the cord is the 

 bundle marked " posterior tracts " in Fig. 23, and this is made 

 up mainly of nerve fibres which start from the cells in the grey 

 matter round which the sensory root fibres terminate. 



The upward path of these bundles is represented very diagram- 

 matically in Fig. 28 as " great sensory tracts from cord to 

 medulla." The individual fibres run up into the medulla, and end 

 there as synapses round cells in two centres or nuclei, thus enter- 

 ing into a second relay. From these centres two new tracts, 

 called the lemnisci, start. Immediately after leaving the nuclei 

 the lemnisci cross or decussate — that is, the fibres starting from 

 the right-hand nuclei cross over to the left-hand side, and 

 vice versa. Running up through the crura of the lower brain, 

 they end in the corpora quadrigemina and optic thalami, and 

 this is their third relay. From these mid-brain nuclei new tracts 

 of fibres start again, and proceed upwards into the cortex. 



Thus the stimuli originating in the sense organs of the skin of 

 the trunk and limbs, and in the muscles, joints, and viscera of 

 those body regions, pass into the spinal cord, ascend the latter 

 to end in the medullary ganglia. From there they travel to the 

 mid-brain ganglia on the opposite sides of the brain, and from 

 the mid-brain they proceed to the cerebral cortex. It is mostly 

 the impulses giving rise to the sensations of touch, heat and cold, 

 and pain originating by stimulation of the skin that take this 

 very complicated path. 



The impulses coming from the receptor organs in the muscles 

 and joints mostly take a different path. They enter the grey 

 matter of the cord via the sensory spinal nerve roots as before, 

 and end in cells from which new ascending fibres start, or they 

 may travel up in the white matter of the cord without first 

 undergoing a relay in the grey matter. But in either case they 

 take an independent course, and proceed to the cerebellum, 



* A " relay " is an interruption in a nervous tract. One set of neurones 

 join on to a new set by means of synapses. 



