CIRCULATION. 



675 



seen to contract on the application of a stimu- 

 lus in the web of the frog's foot by Drs. Thom- 

 son and Hastings. This, however, occurs 

 much more rarely than the contraction of the 

 small arteries. It has been remarked that in 

 some animals muscular fibres are prolonged 

 from the auricle upon the adjoining part of the 

 venacava; and Spallanzani, M.Hall,Flourens,* 

 and others have recorded the fact of the rythmic 

 contraction of parts of the great veins adjoining 

 the auricles. But, excepting in these situations 

 and in the caudal heart, observed by M. Hall 

 in the Eel, muscularity of the veins cannot be 

 considered as having any effect in promoting 

 the flow of the blood in these vessels. 



The progressive motion of the venous blood 

 takes place with little force, and is therefore 

 subject to considerable variations from external 

 pressure. Thus the flow of the blood may be 

 much accelerated by raising a limb, or retarded 

 by keeping it in the depending posture from 

 the mere effect of gravitation, and the common 

 practice of making a person who is bled in the 

 arm call the muscles of the arm into action 

 during the operation, is a sufficient proof that 

 the pressure of the muscles may be the means 

 of accelerating in a considerable degree the 

 venous circulation, an effect obviously depen- 

 dent on the disposition of the valves. Gravita- 

 tion or muscular action are, however, only occa- 

 sional causes of the acceleration of the flow of 

 blood in the veins, and both, but particularly 

 gravitation, may in some instances offer an ob- 

 stacle to its progress. 



There are some physiologists who believe 

 the blood to be drawn through the veins to- 

 wards the heart by a power of suction which 

 operates from the side of the heart or chest. 

 The remarks we have already made in treating 

 of the arterial and capillary circulations render 

 it unnecessary for us to revert in this place to 

 the arguments employed by those who have 

 supported the above view, merely on account 

 of their belief in the inadequacy of the heart's 

 force to maintain the complete circulation ; we 

 shall only now state the direct experiments or 

 reasonings by which it has been attempted to 

 be proved that a vis afronte or suction power 

 draws the blood towards the centre of the cir- 

 culation. We have already, in a former part 

 of this article, stated our reasons for believing 

 that the elastic power of the heart itself is not 

 attended with any production of an appreciable 

 force sufficient to draw the blood into its inte- 

 rior. 



The facts which relate to the supposition 

 that the chest or lungs become, during their 

 motions in respiration, the source of a suction 

 power which acts on the venous blood may be 

 suitably considered under the first part of the 

 fourth division of this article, viz. 



IV. TlIE RELATION OF THE CIRCULATION 

 TO OTHER FUNCTIONS. 



1. Respiration. Of the opinions of those 

 who attribute the suction of the blood through 

 the veins to powers within the chest, there 



* Annalcs dcs Sciences Natur. lorn, xxviii. p. 65. 



are chiefly two which have of late years at- 

 tracted attention, those namely of Dr. Car- 

 son of Liverpool,* and of the late Sir David 

 Barry .f 



According to Dr. Carson the lungs are of a 

 highly elastic nature, and are kept in a state of 

 forced distension by the pressure of the atmo- 

 sphere which enters them when the chest dilates. 

 The lungs would collapse or fall away from the 

 walls of the chest but for the force with which 

 they are distended, and there is thus a tendency 

 to the production of a vacuum within the chest 

 or to a diminution of the pressure on the exte- 

 rior of the heart, in consequence of which the 

 blood is forced or drawn into the heart and 

 chest on the same principle that fluid enters 

 the mouth in the act of sucking. 



According to Sir D. Barry, at each inspira- 

 tion of air into the chest the lungs are not suffi- 

 ciently expanded to fill the whole of the chest, 

 or there is, in consequence of the expansion of 

 the walls of the chest, a less pressure within the 

 chest than on its exterior, and the blood is pro- 

 pelled through the veins communicating with 

 the heart by the external atmospheric pressure. 



Neither Dr. Carson nor Sir D. Barry state, 

 in a sufficiently explicit manner, how much of 

 the force impelling the blood through the veins 

 they conceive to be of the nature of suction : 

 they both admit that the greatest part of this 

 force belongs to the heart or vis a tergo, but 

 they yet state distinctly their belief that the 

 suction power is an important cause of the mo- 

 tion of the blood throughout the whole venous 

 system. The works of both these authors are 

 replete with interesting remarks on the circula- 

 tion in general, and more especially on the flow 

 of blood through the veins. The direct expe- 

 riments, however, in support of their opinions 

 are comparatively few and inconclusive. Dr. 

 Carson shewed that the lungs are always during 

 life iu a state of forced expansion, and estimates 

 the pressure which the lungs of the sheep are 

 capable of sustaining, when in the expanded 

 condition, as equal to a column of seven 

 inches of water. Sir D. Barry observed, in 

 experiments made upon horses, that when 

 one end of a tube is introduced into the ju- 

 gular vein, and the other extremity rests in a 

 vessel containing water, the water rose during 

 each inspiration some length in the tube, and 

 sank again during expiration, distinctly indi- 

 cating the diminished pressure existing within 

 the chest at the time of the rise of the water, 

 and proving that the flow of the blood in some 

 parts of the veins may be accelerated during 

 inspiration from the same cause. Poiseuille,J 

 by the employment of the instrument for mea- 

 suring the pressure of the animal fluids, to which 

 allusion has already frequently been made, has 

 confirmed Sir D. Barry's statement, that the di- 

 minished pressure within the chest, at the time 

 of inspiration, is such as to affect the flow of 



* Inquiry into the Causes of the Motion of the 

 Blood, &c. Liverpool, 1815. 



t Experimental Researches on the Influence of 

 Atmospheric Pressure upon the Progression of the 

 Blood in the Veins, &c. Lond. 1826. 

 Loc. citat. 



