286 SCIENCE PROGRESS 



supposed to be represented in the human body by the " animal 

 spirits," described as a kind of " subtle gas," or rather a "very 

 active and very pure flame," originating in the heart and deriving 

 its energy from the heat of that organ. This is mingled with the 

 blood and carried by an artery to the pineal gland, where it is 

 separated again and poured out as from a fountain into the 

 cavities of the brain. The inner surface of these cavities is 

 supposed to be porous, and the pores are supposed to lead into 

 the nerve fibres, which are represented as being hollow tubes 

 down which the animal spirits flow. Thus what we now call 

 motor impulses were thought by Descartes to be due to the 

 actual movement of a quantity of fluid from the brain cavity 

 through the nerves to the muscles, which they cause to move by 

 rendering them turgid. 



Meanwhile the soul sits in the pineal gland, like the engineer 

 of the water-works in the midst of his pipes and fountains, and 

 exercises a kind of general supervision — influencing the animal 

 spirits in various ways ; suffering pain when things go wrong 

 and rejoicing when they go right. 



The general belief in the fundamentally azygos or unpaired 

 nature of the pineal gland, to which Descartes attributed such 

 unique importance, was destined for many years to prejudice 

 opinion as to the true character and morphological relations of 

 this organ. Moreover, our knowledge was for a long time 

 derived almost exclusively from the human subject, in which the 

 pineal gland exhibits the highest degree of secondary modifica- 

 tion, and has lost almost every trace of its primitive character ; 

 and it was not until the problem was attacked by comparative 

 anatomists that any real advance was made in this direction. 



In 1872 Leydig, apparently for the first time, called attention 

 to a " parietal sense-organ " which he had discovered in lizards 

 (Lacerta) and blind-worms {Anguis). Ten years later Rabl- 

 Ruckhard began to speculate on the possible homology of this 

 organ with the median eyes of arthropods, and in 1884 the 

 same author called attention to the unpaired foramen which 

 exists in the parietal region of the large fossil reptiles of the 

 secondary period, e.g. ichthyosaurians and plesiosaurians, and 

 suggested that this foramen might have contained the modified 

 distal extremity of the pineal organ, in the form of a sense-organ 

 which served to warn its possessor of the intense heat of the 

 tropical sun. In the same year Strahl also maintained that 



