SENSORY, MOTOR & PSYCHIC NEURONS 409 



columns of co-related histological extensions of the upper 

 cerebro-spinal axons, and the succeeding spinal axons, 

 sensory and motor, as they are given off or received by 

 the various spinal nerve trunks ; the gross quantity of 

 the grey matter undergoing increase where the great 

 plexuses are formed for the innervation of the upper and 

 lower limbs respectively, and where necessarily the glial 

 substance must be present in increased quantity to meet 

 the nutritional wants of the increased neuronal expendi- 

 ture, sensory and motor, or cutaneous and muscular. 

 The contrasting arrangements as to position of the 

 intra-cephalic and intra-spinal (see Fig. 131) grey and 

 white matter are very striking, and seem to suggest 

 a " natural selection " of a profound character, as being 

 the determining influence such that the true neuronal, 

 or nerve cell, areas require an environment of the most 

 protected and insulated character possible, and that nature 

 procured these wherever she could consistently with 

 material necessity and functional advantage. Thus, 

 immediately under the protective influence of the skull 

 meninges and the cerebro-spinal fluid, as well as under 

 the more resistant local structural developments and 

 arrangements of the white, or connecting and conducting, 

 substance of the brain, and within a complete encasement 

 of resistant white matter in the spinal cord, nature has 

 sought to procure protection and support, for perhaps 

 the most invaluable, as well as the most highly functioned 

 and important structures of the body, and so has once 

 more displayed a marvellous adaptation of means to 

 ends, and an example of design which calls for recog- 

 nition, as one of the most outstanding, and embryonically 

 the latest, in the whole category of such " natural 

 wonders." 



