BRAIN AND NERVE 105 



and more developed in the dog, and still more in the monkeys. 

 In the anthropoid apes and in man they attain the maximal 

 development, and are to be regarded as the paths along which 

 those impulses travel that are the stimuli to what we call 

 " willed " or spontaneous actions and movements. 



This is a very summary account of the main features of the 

 gross anatomy of the central nervous system, and we deal in 

 greater detail later with the special nervous mechanisms. The 

 reader should now be able to visualise the whole as a series of 

 parts developing progressively in the course of the evolution of 

 the vertebrate animals. Primarily there was a double series of 

 ganglia, one pair in each segment of the body, and all of them 

 were connected together by transverse and longitudinal com- 

 missural tracts. Nerves issuing from the ganglia were dis- 

 tributed to the sensory and motor organs of the body, conducting 

 inwards impulses originating in the stimulation of the receptors, 

 and conducting outwards impulses setting the muscles in move- 

 ment or causing glands to function. With increasing complexity 

 of bodily structure and greater freedom of movement the ganglia 

 increased in mass and began to coalesce, thus forming the con- 

 tinuous core of grey matter of the cord, the hind, mid, and fore 

 brain. The great sense organs became concentrated in the head, 

 and so the cranial ganglia increased in functional importance, 

 finally assuming the conditions that have been described. 



There has been a progressive complexity in the movements 

 included in locomotion as we ascend the series of evolutionary 

 phases represented by the fishes, amphibia, reptiles, birds, and 

 mammals. The need for precise adjustment and co-ordination 

 of the impulses issuing from the spinal cord and setting muscular 

 organs in motion therefore led to the development of specialised 

 ganglia carrying out such functions, and so the cerebellum 

 assumed the anatomical importance that it obviously has in the 

 human brain. 



And in the later stages of this evolution that is, in the 

 mammals a kind of activity, connoted by the terms " intel- 

 ligent," " spontaneous," and " volitional," became the charac- 

 teristic one exhibited by these animals. This, we shall see, 

 involved a nervous mechanism other than those of the mid- 

 brain and cerebellum, which are to be regarded as the ganglia 

 controlling movements and activities that are largely " auto- 

 matic." This mechanism, the latest one to be evolved, is 

 contained in the cortex cerebri and its connections. 



