AN ENQUIRY INTO THE EORMATION OF HABIT IN MAN. 135 



I would also finally ask the forbearance of any physiolo- 

 gists that may be present, if in order to make the subject 

 clear to those who have not pursued these studies, I preface 

 my remarks Avith a brief summary of the general arrange- 

 ment and structure of nerve tissues. 



Brain structure generally. — The adult brain in man weighfi 

 between 46 and 53 ounces, with extreme limits from 23 ounces 

 in an idiot to Qb ounces in men of the highest attainments. In 

 women brains weigh between 41 and 47 ounces. The brain 

 of the highest anthropoid ape weighs about 15 ounces. The 

 brain is in two halves, right and left, and in four portions : 

 the hemispheres and cortex, the seat of purely intelligent 

 and voluntary actions ; the middle brain, consisting of large 

 gangha, whence proceed the ordinary movements of animal 

 life, not necessarily voluntary ; behind this the cerebellum, 

 or little brain, co-ordinating the movements, especially those 

 connected with the erect position ; and below, the medulla, 

 which contains nearly all the involuntary centres connected 

 with the maintenance of passive physical life. 



The cortex, which increases in size in animals in proportion 

 to the rest of the brain, in the ratio that intelligence super- 

 sedes instinct, is covered or rather composed of convolutions 

 Avhich by their number and depth afford a very fair idea of 

 the amount of intellectual development. They increase 

 steadily in intricacy as Ave ascend the animal scale ; they also 

 increase in man up to fifty years of age, after Avhich they get 

 gradually less marked ; the brain as a Avhole also decreasing in 

 Aveight about one ounce OA^ery ten years. The brain and spmal 

 cord are each pierced Avitli a continuous central tube sur- 

 rounded with grey or nerve cell matter, AA'hich in its turn is 

 surrounded by Avhite or nerA^e fibi-e matter. The cortex or 

 surface of the brain is coA^ered to the depth of about a 

 quarter of an inch Avith another layer of grey cell matter, 

 the superficial extent of Avhicli is obviously greatly increased 

 by the couA^olutions. 



The brain is continually Avasting and being repaired, the 

 ncAv tissue always accurately reproducing all the features of 

 the old, Avliether these be congenital or acquired. 



Nerve structure generally. — The nerve centres consist of three 

 main elements ; nerve cells, nerve fibres, and the groundwork 

 or AvebAvork in AA'hich both are embedded, called neuroglia. 

 In the cortex this substance looks like ground glass, and 

 imder a A'ery high poAA^er is seen to be traversed in every 

 direction Avith very fine AAdiite fibres less than jq/itttt <^f ^^^ 



