CH. II.] CEREBRAL IMPRESSIONS. 67 



destined solely to the transmission upwards of external im- 

 pressions made on the terminating fibrils (perhaps the vital 

 spirits being present in them), while on the other hand, other 

 fibrils are destined solely to the transmission downwards to the 

 terminating fibrils of internal impressions on the brain (perhaps 

 the vital spirits being present at the cerebral origin), just as 

 there are two classes of blood-vessels having similarly opposed 

 functions ? According to this analogy (which comes nearest to 

 the idea of Borellus (^De Motu Anim.,^ § 159, and which 

 A. Monro does not entirely disprove), the brain that produces 

 the vital spirits sends them downwards through certain fibrilli 

 in a nerve to the terminal points (the sensitive papillae), where 

 they are received by the terminating points of other nerves, and 

 transmitted back to the brain, as if to a heart. Although 

 this theory cannot be fully demonstrated, it has a great degree 

 of probability, since by it we can comprehend many phenomena 

 which otherwise would remain incomprehensible. It will be 

 worth while to render this matter more explicit. 



127. When an external expression has been made on a 

 nerve on the surface of the body, it passes upwards if there be 

 no impediment (13), and reaches the brain at the point of origin 

 of the nerve. The other nerve-fibrils, which cannot transmit 

 the impression upwards, are not affected by the influence from 

 without. At the point of origin of the nerve in the brain, 

 the transmitted external impression produces a material idea 

 which excites an external sensation. By this material idea (a 

 movement) at the origin of the nerve, those fibrils are impressed 

 that propagate the internal impression downwards; whilst on 

 the other hand, those fibrils that transmitted the external im- 

 pression to the brain, receive no impression from the external 

 sensation. The former, however, propagate the conceptional 

 impression to those structures, in which the sentient actions of 

 the external sensation can arise, and these result accordingly. 

 It is now more intelligible, how these opposite movements, 

 arising from opposite impressions in the same nerve, are not 

 impeded by each other, and why the same nerve can, at the 

 same time that it transmits an external impression to the 

 brain, produce a sentient action in some organ of the body, as 

 for example, a voluntary movement. Since a nerve in its 

 course from the brain is divided into many branches, which 



