CIRCULATION. 675 



seen to contract on the application of a stimu- are chiefly two which have of late years at- 



lus in the web of the frog's foot by Drs. Thorn- tracted attention, those namely of Dr. Car- 



son and Hastings. This, however, occurs son of Liverpool,* and of the late Sir David 



much more rarely than the contraction of the Barry .f 



small arteries. It has been remarked that in According to Dr. Carson the lungs are of a 



some animals muscular fibres are prolonged highly elastic nature, and are kept in a state of 



from the auricle upon the adjoining part of the forced distension by the pressure of the atmo- 



vena cava; and Spallanzani, M.Hall,Flourens,* sphere which enters them when the chest dilates, 



and others have recorded the fact of the rythmic The lungs would collapse or fall away from the 



contraction of parts of the great veins adjoining walls of the chest but for the force with which 



the auricles. But, excepting in these situations they are distended, and there is thus a tendency 



and in the caudal heart, observed by M. Hall to the production of a vacuum within the chest 



in the Eel, muscularity of the veins cannot be or to a diminution of the pressure on the exte- 



considered as having any effect in promoting rior of the heart, in consequence of which the 



the flow of the blood in these vessels. blood is forced or drawn into the heart and 



The progressive motion of the venous blood chest on the same principle that fluid enters 

 takes place with little force, and is therefore the mouth in the act of sucking, 

 subject to considerable variations from external According to Sir D. Barry, at each inspira- 

 pressure. Thus the flow of the blood may be tion of air into the chest the lungs are not suffi- 

 much accelerated by raising a limb, or retarded ciently expanded to fill the whole of the chest, 

 by keeping it in the depending posture from or there is, in consequence of the expansion of 

 the mere effect of gravitation, and the common the walls of the chest, a less pressure within the 

 practice of making a person who is bled in the chest than on its exterior, and the blood is pro- 

 arm call the muscles of the arm into action pelled through the veins communicating with 

 during the operation, is a sufficient proof that the heart by the external atmospheric pressure, 

 the pressure of the muscles may be the means Neither Dr. Carson nor Sir D. Barry state, 

 of accelerating in a considerable degree the in a sufficiently explicit manner, how much of 

 venous circulation, an effect obviously depen- the force impelling the blood through the veins 

 dent on the disposition of the valves. Gravita- they conceive to be of the nature of suction : 

 tion or muscular action are, however, only occa- they both admit that the greatest part of this 

 sional causes of the acceleration of the flow of force belongs to the heart or vis a tergo, but 

 blood in the veins, and both, but particularly they yet state distinctly their belief that the 

 gravitation, may in some instances offer an ob- suction power is an important cause of the mo- 

 stacle to its progress. tion of the blood throughout the whole venous 



There are some physiologists who believe system. The works of both these authors are 



the blood to be drawn through the veins to- replete with interesting remarks on the circula- 



wards the heart by a power of suction which tion in general, and more especially on the flow 



operates from the side of the heart or chest, of blood through the veins. The direct expe- 



The remarks we have already made in treating riments, however, in support of their opinions 



of the arterial and capillary circulations render are comparatively few and inconclusive. Dr. 



it unnecessary for us to revert in this place to Carson shewed that the lungs are always during 



the arguments employed by those who have life in a state of forced expansion, and estimates 



supported the above view, merely on account the pressure which the lungs of the sheep are 



of their belief in the inadequacy of the heart's capable of sustaining, when in the expanded 



force to maintain the complete circulation ; we condition, as equal to a column of seven 



shall only now state the direct experiments or inches of water. Sir D. Barry observed, in 



reasonings by which it has been attempted to experiments made upon horses, that when 



be proved that a vis afronte or suction power one end of a tube is introduced into the ju- 



draws the blood towards the centre of the cir- gular vein, and the other extremity rests in a 



culation. We have already, in a former part vessel containing water, the water rose during 



of this article, stated our reasons for believing each inspiration some length in the tube, and 



that the elastic power of the heart itself is not sank again during expiration, distinctly indi- 



attended with any production of an appreciable eating the diminished pressure existing within 



force sufficient to draw the blood into its inte- the chest at the time of the rise of the water, 



rior. and proving that the flow of the blood in some 



The facts which relate to the supposition p ar t s of the veins may be accelerated during 



that the chest or lungs become, during their inspiration from the same cause. Poiseuille,J 



motions in respiration, the source of a suction by the employment of the instrument for mea- 



power which acts on the venous blood may be suring the pressure of the animal fluids, to which 



suitably considered under the first part of the allusion has already frequently been made, has 



fourth division of this article, viz. confirmed Sir D. Barry's statement, that the di- 

 minished pressure within the chest, at the time 



IV. THE RELATION OF THE CIRCULATION o f inspiration, is such as to affect the flow of 



TO OTHER FUNCTIONS. 



1. Respiration. Of the opinions of those ' In <l uir y V n . to the 1 C J*J of the Motlon 



who attribute the suction of the blood through B \t; p ^i m ent V aT P RetarcJf; on the Influence of 



the veins to powers within the chest, there Atmospheric Pressure upon the Progression of the 



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



* Annales des Sciences Natur. torn, xxviii. p. 65. t koc. "tat. 



