288 



itself, with difficulty, into the right ventricle, whose blood is unable to 

 penetrate through the pulmonary tissue. On the other hand, as the lungs 

 compress the vena cava, a regurgitation takes place of the blood which 

 it was conveying to the heart; forced back along the jugulars and verte- 

 brals, it distends these vessels, the sinus of the dura mater which empty 

 themselves into them, and the veins of the brain which terminate in 

 these sinuses. Their distension accounts for the elevation of the cerebral 

 mass soon followed by depression, when, on inspiration succeeding expi- 

 ration, and on the lungs dilating, the blood which fills the right cavities 

 of the heart can freely penetrate into the pulmonary substance, and make 

 way for that which the vena cava is bringing from the superior parts of 

 the body. 



Haller considered this reflux as very difficult, the blood having to rise 

 against its own gravity; and he admitted Lamure's explanation only in 

 the forcible acts of respiration, as in coughing, laughing, and sneezing. 

 He maintained that, in a state of health, there is to be observed, during 

 expiration, a mere stagnation of the blood, in the vessels which bring it 

 from the internal parts of the skull. He further admits, on the testimony 

 of a great number of authors, another order of motions depending on 

 the pulsations of its arteries; so that, according to Haller, the cerebral 

 mass is incessantly affected by motions, some of which depend on respi- 

 ration, while others are quite independent of it. 



Lastly, according to Vicq-d'Azyr, the brain, on being exposed, pre- 

 sents a double motion, or rather, two kinds of motion, from without; the 

 one from the arteries, and, which is least remarkable, the other from the 

 alternate motions of respiration. 



CXLIX. This opposition between authors of reputation, and whose 

 theories have, in general, been adopted, induced me to repeat the experi- 

 ments which each of them brings in support of his own opinion, and to 

 perform further experiments on this subject. My investigation soon con- 

 vinced me, that these authors had given a statement of their opinions, and 

 not of the fact itself. In fact, the alternate motions of elevation and 

 depression observed in the brain, are isochronous to the systole and dia- 

 stole of the arteries at its base. The elevation of the brain corresponds 

 to the dilatation of these vessels, its depression to their contractions. 

 The process of respiration has nothing to do with this phenomenon, and 

 even admitting the stagnation of the regurgitation of the blood in the 

 jugular veins, the arrangement of the veins, within the skull, is such, that 

 this stagnation or reflux could not produce alternate motions of the cere- 

 bral mass. 



The brain receives its arteries from the carotids and vertebrals, after 

 they have entered the skull, the former along the carotid canals, the lat- 

 ter through the foramen magnum of the occipital bone. It would be use- 

 less to describe their numerous divisions, their frequent anastomoses, the 

 arterial circle, or rather polygon formed by these anastomoses, and by 

 means of Avhich the carotid and vertebral arteries communicate together, 

 by the side of the sella turcica. Haller has given a very correct view and 

 an excellent description of this part*. The account of the internal caro- 

 tid artery published by that great anatomist is, according to Vicq-d'Azyr, 

 a chief-d'oeuvre of learning and precision ; the same encomium might be 



Fasciculi Anatomici, F. 7, tab. 2, 



