CH. II.] CEREBRAL FORCES. 28 



facility of flux and reflux which it acquires. In like manner it is 

 probable that prolonged watching, starvation, debilitating food 

 and drugs, the emotions, and the active elements of certain 

 matters which, from their destructive qualities, are injurious to 

 the nervous fluid, as opium, for example; also cold, indolence, 

 want of exercise, fatigue, vexation, intense application of the 

 mind or of the senses, all interrupt or diminish the animal 

 forces, because they either diminish the vital spirits, or impede 

 their secretion, or render them morbid, or hinder their flux and 

 reflux. 



Note. — Although little is known of the nature and proper- 

 ties of the forces of the vital spirits, the physician can content 

 himself therewith, even although the little that we think we 

 know is doubtful, and at the best only probable : for they may 

 remain undetermined for ever without any loss to science, be- 

 cause we are under no necessity to show the origin and nature 

 of the animal forces, inasmuch as we learn their true actions 

 and laws from observation only. 



SECTION II. THE FORCES OF THE BRAIN CONSIDERED 



ABSTRACTEDLY AS ANIMAL-SENTIENT FORCES. 



24. The brain has a regular double movement, which is 

 mechanical only, and not peculiar to its animal nature. One 

 movement is simply the motion communicated to it by the 

 arteries; the other consists in an alternate eff'ort to expand 

 and contract, which Haller attributes to the connection between 

 the respiration and the cerebral veins, so that the latter, like 

 the brain itself, become turgid at each expiration, and fl accid 

 at every inspiration (Haller's ^ Physiology^). Although this me- 

 chanical motor power of the brain, as well as the consequent 

 secretions, together with the cerebral circulation and its purely 

 physical forces, do not properly come under our notice here, 

 but belong to the physiology of the mechanical nature of animal 

 organisms, still it is necessary to remember them in an inquiry 

 into its animal forces, so far as the existence of the latter pre- 

 supposes their existence. Since respiration is the cause of the 

 continual movements of the brain just mentioned, and without 

 it, indeed, the animal-sentient forces cannot act, because their 

 action presupposes the existence of the mechanical forces (6), 



