384 OF THE CIRCULATION OF BLOOD. 



retarded in the systemic capillaries, causing an increased pressure on the walls 

 of the arteries. He found that, when the Ingress of air through the trachea 

 of a dog was prevented, and the Asphyxia was proceeding to the stage of in- 

 sensibility, the attempts at inspiration being few and laboured, and the blood 

 in an exposed artery being quite venous in its character, the pressure upon 

 the arterial walls, as indicated by the haemadynamometer applied to the femoral 

 artery, was much greater than usual. Upon applying a similar test to a vein, 

 however, it was found that the pressure was proportionably diminished ; whence 

 it became apparent, that there was an unusual obstruction to the passage of 

 venous blood through the systemic capillaries. After this period, however, 

 the mercury in the hsemadynamometer applied to the artery began to fall stea- 

 dily, and at last rapidly, in consequence of the diminished force of the heart, 

 and the retardation of the blood in the pulmonic capillaries ; but, if atmo- 

 spheric air was admitted, the mercury rose very speedily, showing that the 

 renewal of the proper chemical state of the blood restored the condition neces- 

 sary for its circulation through the capillaries. 



513. It can be scarcely doubted, that it is by some influence exercised over, 

 the molecular actions, to which the Blood is subject in the Capillaries, that the 

 Nervous system can operate on the functions of Nutrition, Secretion, &c., in 

 the manner already alluded to (Chap, vn.) ; and this influence may be not 

 improperly termed vital, if by so designating it we merely imply that its 

 nature and mode of operation are unknown, but that it is closely connected 

 with those actions which are altogether peculiar to living beings. The fol- 

 lowing experiment, made by Dr. Wilson Philip, exhibits, in a convincing 

 manner, the possibility of such an influence. " The web of one of the hind 

 legs of a frog was brought before the microscope ; and while Dr. Hastings 

 observed the circulation, which was vigorous, the brain was crushed by the 

 blow of a hammer. The vessels of the web instantly lost their power, the 

 circulation ceasing; an effect which cannot arise, as we have seen, from the 

 ceasing of the action of the heart. [Dr. P. here refers to experiments, by 

 which it was ascertained that the circulation in the capillary vessels of the 

 frog will continue for several minutes after the interruption of the heart's 

 action.] In a short time the blood again began to move, but with less force. 

 This experiment was repeated with the same result. If the brain is not com- 

 pletely crushed, although the animal is killed, the blow, instead of destroying 

 the circulation, increases its rapidity."* We are not hence to conclude, how- 

 ever, that the Nervous system supplies any influence which is essential to the 

 continuance of the Circulation ; since it is only by such sudden and severe 

 injuries to the nervous centres, as instantaneously destroy the vitality of the 

 whole system ( 386), that the movement of the blood is arrested. The experi- 

 ments of Miiller and others satisfactorily prove, that mere action of the Nerves 

 does not produce any direct effect upon the Capillary circulation ; and this 

 corresponds with the well-known fact, that the Nutritive processes may con- 

 tinue as usual, after this action has been suspended. All the facts which bear 

 upon the question of the connection between nervous agency and the Capil- 

 lary Circulation, have an equal relation to the functions of Nutrition and 

 Secretion in general. 



IV. Of the Venous Circulation. 



514. The Venous system takes its origin in the small trunks that are 

 formed by the re-union of the Capillaries ; and it returns the blood from these 

 to the Heart. The 'structure of the Veins is essentially the same with that of 



^Experimental Inquiry into the Laws of the Vital Functions, 4th edition, p. 52. 



