CIRCULATION. 



655 



In the Planaria, Medusa, some Entozoa, 

 and Polypi, the subdivided or ramified cceca 

 of the alimentary cavity (fig 328, I) must 

 obviously contribute to the effect of furnishing 

 a supply of digested matter to the different 

 regions of the body, and of thus rendering a 

 distinct vascular system in them to a certain 

 extent unnecessary. But in these simpler 

 kinds of animals, and even in those of them 

 in which distinct vessels have been discovered, 

 we cannot regard such scattered tubes as the 

 only principal means of distributing the nutri- 

 tious fluids to the different parts of the body. 

 They may assist in bringing this about ; but it 

 is also necessary to suppose the occurrence of 

 an interstitial movement or organic transuda- 

 tion of the fluids, in order to furnish to all 

 the parts the materials for assimilation. 



III. PHENOMENA OF THE CIRCULATION AND 



POWERS MOVING TIIE BLOOD. 



In proceeding to the third division of our 

 subject, viz. the phenomena of the circulation 

 and the powers by which the blood is moved, we 

 would remark, that, however desirable it might 

 appear in a systematic work of this kind to treat 

 of these two subjects under distinct heads, such 

 a separation would have the effect of detaching 

 inconveniently the facts from the legitimate 

 conclusions which may be drawn from them. 

 We shall first state the phenomena and causes 

 of the motion of the blood which belong strictly 

 to the organs of circulation themselves, and 

 afterwards shall treat of various circumstances 

 connected with the other functions by which 

 the circulation is modified. In this view it is 

 our chief object that the facts adduced should 

 bear upon the explanation of the motion of the 

 blood in the human body, but from the nature 

 of the investigation the facts themselves must 

 be drawn chiefly from experiments made upon 

 the lower animals. Of course those experi- 

 ments and observations which have been made 

 on Mammiferous animals have most value in 

 relation to such a view of the function as that 

 which it is our intention to give. The order 

 which we shall follow is founded on the course 

 which the blood pursues. We shall treat, 1, 

 of the passage of the blood through the heart; 

 2, of its flow in the aiteries ; 3, of its. passage 

 from the arteries to the veins through the ca- 

 pillaries; and 4, of its flow in the veins. 



1. Flow of the blood through the heart. 

 That the muscular contraction of the heart is, 

 in man and in all animals in which this organ 

 exists, the principal source of the power by 

 which the blood is propelled in its course, 

 seems to be satisfactorily proved by the facts, 

 that whenever the action of the heart ceases or 

 is impeded, the whole circulation ceases, and 

 that, when an obstruction prevents the action 

 of the heart from reaching the blood in any of 

 the bloodvessels, the flow of blood ceases almost 

 instantaneously in all the branches proceeding 

 from the obstructed vessel. The constant and 

 regular persistence of the contractions of this 

 muscular organ from the commencement of life 

 to its termination, the early period at which it 

 begins to act in the foetus, viz. before any re- 



gular circulation of blood takes place, and the 

 existence of a heart or some similar contractile 

 organ in all those animals in which a regular 

 circulation of blood or nutritious fluids occurs, 

 are confirmatory of the view suggested by direct 

 observation and experiment. Under the article 

 HEART will be found a detailed account of the 

 structure and functions of this organ ; in this 

 place we shall only state, in as few words as 

 we can, what seems to have been best ascer- 

 tained regarding its action, in so far as this 

 appears to have a reference to the force of im- 

 pulsion and direction which it communicates 

 to the blood. 



The action of the heart may be observed by 

 opening the chest of a living animal, or for a 

 short time in one immediately after death, or 

 best of all in an animal deprived of sense and 

 motion by poison, and in which artificial respira- 

 tion is maintained ; it has also been seen in chil- 

 dren born with ectopia cordis, or in persons in 

 whom from accident a part of the heart has 

 been exposed to view. When observed under 

 one or other of these circumstances, the action 

 or contraction of the whole heart is seen to 

 consist of two motions, viz. 1, the contraction 

 or systole of the auricular part, and 2, that of 

 -the ventricular part of the organ. The con- 

 traction of the auricle immediately precedes 

 that of the ventricle and seems to be continued 

 into it, and the systole of each cavity is imme- 

 diately followed by its diastole or relaxation.* 

 After the relaxation of the ventricle, there is a 

 period of repose, or a pause in the action of 

 the heart, during which motion seems to be 

 nearly suspended. At the moment when the 

 systole of the ventricle takes place, the heart 

 appears to be diminished in all its dimensions, 

 and exactly at the same instant of time, the 

 apex is seen to be moved towards the sternum, 

 in whatever position the animal is placed. 

 This tilting forwards of the apex gives the 

 heart a pulsation against the ribs that can be 

 felt externally. This pulsation probably de- 

 pends on the arrangement of the muscular 

 fibres of the heart, as the raising of the apex 

 occurs when the heart is removed from the 

 body and is empty of blood. At the time 

 of the systole the heart is thicker and more 

 conical in its figure than during the diastole; 

 when held in the hand it feels hard, and the 

 ventricles appear to have propelled the whole 

 of the blood out of their interior, as far as one 

 can judge from the great diminution in their 

 size. In the inferior animals, as Reptiles and 

 Fishes, its colour is lighter from the expulsion 

 of the blood. During the relaxation or dias- 

 tole, the heart appears to fall away from the 



c In some of the lower animals, in the foetus 

 of the Bird at an early period, and in warm- 

 blooded animals when the action of the heart is 

 weakened, as at the approuch of death, the con- 

 traction is seen to begin in the venous sinus of 

 the auricle, extend through ii to the ventricle, and 

 from one part of the ventricle to another in a gra- 

 dual manner. In the Batrachia, the contraction 

 begins in the veins, and after passing through the 

 auricle and ventricle, extends into the commence- 

 ment of the aorta. 



