THE PINEAL GLAND 285 



the passage by which the " spirits " of its anterior cavities 

 communicate with those of its posterior cavities, in such a 

 manner that any movement in the gland will alter the course 

 of these spirits, while at the same time any alteration in the 

 course of the spirits will affect the movements of the gland. 



There can be no doubt that the gland here referred to, 

 though not mentioned by any special name, is the pineal gland 

 or conarium of human anatomists, although the author's limited 

 acquaintance with the structure of the human brain caused him 

 to represent it as lying inside instead of outside the brain-cavity. 



Descartes goes on to explain that the reason which induced 

 him to regard this gland as the place where the soul immediately 

 exercises its functions is that the other parts of the brain, as 

 well as the external organs of sense, are all double, while our 

 thoughts are single, i.e. we only have one single and simple 

 thought of one and the same thing at the same time. Hence the 

 double impressions which come from the paired organs of sense 

 — such as the two images from the two eyes — must be able to 

 unite into one before they reach the soul, or else we should see 

 two objects instead of one ; and this union is supposed to take 

 place in the pineal gland by the mediation of the spirits which 

 fill the cavities of the brain. 



In a later essay, entitled L Homme, Descartes elaborates his 

 peculiar system of physiology, and it is most curious to note 

 the enormous importance which he attributes to what he terms 

 " the little gland H," shown in his charmingly quaint illustrations 

 as a conspicuous pear-shaped body lying in the middle of the 

 brain, or sometimes simply in the middle of the head, the rest 

 of the brain being omitted. 



In the seventeenth century there were commonly to be seen 

 in the royal gardens or parks in France elaborate mechanical 

 toys worked by means of water carried in pipes from a central 

 spring to the various figures or other pieces of mechanism which 

 it was desired to set in motion. These contrivances seem to 

 have furnished Descartes with his principal ideas on the 

 physiology of the nervous and muscular systems. Thus he com- 

 pares the " external objects " which act upon the sense-organs, 

 and thereby cause the body to execute movements of various 

 kinds, to strangers who enter unawares into one of the surprise 

 grottoes, and by treading upon the concealed machinery set the 

 figures in motion. The water which works the apparatus is 



