CH. III.] NERVES OF BLOOD-VESSELS. 85 



brain (headache), change the circulation of the blood, and some- 

 times distend, sometimes empty, the vessels of the head, and 

 render the colour of the face so variable. All this it is 

 true is conjecture, but it is probable; we cannot decide the 

 question, for proper observations as to the mode are impossible. 

 We see, indeed, that some species of conceptions regularly 

 and powerfully modify the vital movements, and especially the 

 circulation within the head ; but still since this may probably 

 take place through the nerves distributed to the heart (for the 

 impressions of such conceptions influence its movements by 

 means of its nerves), it remains undetermined whether the 

 direct action on the cerebral capillaries of the cerebral im- 

 pression has a share in these sentient actions, and to what extent. 



Actions of the Material Ideas in the Mechanical Machines 

 through the Nerves. 



160. The most common and most general mode of connec- 

 tion between the nerves and mechanical machines is the rami- 

 fication and subdivision of the former in the latter, until they 

 become imperceptible (13). Another is, however, possible : the 

 nerves come in their course into contact with various me- 

 chanical machines, without being specially distributed to or 

 lost in them. As the cerebral impression on the nerve pro- 

 duces no visible or observable movement at the point of im- 

 pression, so neither in traversing the nerve does it excite any 

 action in the parts with which in its direct course it comes into 

 contact. But as it is probable, that its deflection at the 

 points where it is turned from its direct course, is caused 

 y a vivid cerebral impression (151) ; it is probable, also, in 

 ch a case it may communicate some movement to the me- 

 chanical machines it comes into contact with at such points, 

 and this may be termed a sentient action (97). This conjecture 

 is very probable as to the loops of nerves, which wind round 

 numerous blood-vessels, and enclose them, certainly not without 

 an object. Probably, many vivid cerebral impressions slightly 

 compress the vessels by this means, and so retard the circulation, 

 as Haller has supposed, who by this hypothesis has explained the 

 redness of the face in blushing. {' Opera Minora,' tom. i, p. 513.) 

 Subsequently, however, this distinguished man discarded the 



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