640 



CIRCULATION. 



the pulmonary veins (/?). The left auricle (A) 

 receives the newly arterialized blood from the 

 pulmonary veins, and transmits it to the left 

 ventricle (-BTJ, from which it is ready to start 

 again, when the ventricle contracts, on the 

 same course as has just been described. 



In this double circulation, the path which 

 the blood traverses in passing from the left to 

 the right side of the heart through the aortic 

 arteries and the corresponding veins, has been 

 ' called the greater or systemic circulation : and 

 the route of the blood from the right to the left 

 side of the heart through the pulmonary arte- 

 ries and veins has been termed the Lesser or 

 pulmonic circulation. The names of pulmonic 

 and systemic, indicating tfye parts of the body 

 in which each of these circulations respectively 

 occurs, are on the whole preferable to the cor- 

 responding terms of lesser and greater. 



There is still one part of the course of the 

 blood to be mentioned, viz. that of the venous 

 blood of the principal abdominal viscera 

 through the liver, or what has been termed the 

 system of the vena portse. 



The blood supplied by the coeliac and me- 

 senteric arteries (I, i) to the abdominal viscera 

 1 is not returned directly to the heart by their 

 corresponding veins, as occurs in other parts 

 of the body. The veins of the stomach and 

 intestinal canal, of the spleen, pancreas, me- 

 sentery, omenta, and gall-bladder, unite to- 

 gether below the liver into one large vessel 

 (), the trunk of the vena portae, which 

 branches out again and distributes to the liver 

 by its ramifications the whole of the venous 

 blood coming from the above-mentioned organs. 

 The blood of the vena portse, being joined in 

 the minute branches by that of the hepatic 

 artery (/*), passes into the smallest ramifica- 

 tions of the hepatic veins, by the principal 

 trunks of which (/), the venous and arterial 

 blood circulated through the liver is carried to 

 the inferior vena cava, and thus reaches at last 

 the right side of the heart. 



Proofs of the circulation.-* After this brief 

 outline of the course which the blood takes 

 through the circulatory organs in man and 

 warm-blooded animals, it may be proper to 

 introduce an enumeration of those circum- 

 stances which are generally adduced as af- 

 fording the most satisfactory " proofs of the 

 circulation" or evidence that the blood pursues 

 the paths above detailed. 



As proofs of the circulation, besides those 

 derived from the connection of the different 

 orders of great vessels with the cavities of the 

 heart to which they are respectively attached, 

 may be mentioned 



1st. The structure and disposition of the 

 auriculo-ventricular valves of the heart, and 

 semilunar valves of the aorta and pulmonary 

 artery, which admit of the passage of blood 

 from the auricles to the ventricles, and from 

 the latter cavities to the great arteries, but not 

 in a reverse direction. 



2nd. The mechanism of the valves of the 

 systemic veins which allow of the motion of 

 fluid only in the direction towards the heart. 



3rd. The fact that when a ligature is applied 



to an artery, or any other impediment opposed 

 to the free passage of blood through if, the 

 vessel becomes dilated on the side next the 

 heart, while the application of a ligature to the 

 trunk of a vein is followed by a turgescence of 

 the vessel beyond the place where the obstruc- 

 tion occurs. 



4th. That on opening one of the larger arteries, 

 blood issues in a jet from the end next to the 

 heart at the time of every contraction of that 

 organ, and that in general no blood flows from 

 the orifice of the remote part of the artery: 

 and that on opening a vein the converse is ob- 

 served, the blood issuing freely in a continued 

 stream from the remote part, but none proceed- 

 ing from the part of the vein adjoining the 

 heart. 



5th. That the passage of the blood from the 

 arteries to the veins in the small or capillary 

 vessels has been observed by means of the 

 microscope in transparent parts of animals, 

 and, though it has not been seen in man, we are 

 entitled from the general analogy in the struc- 

 ture of the organs of circulation to infer that 

 the same passage occurs in the human body. 



6th. That, by mechanical arrangements, 

 fluids may easily be made to pass in the dead 

 body through the whole course of the double 

 circulation, but not in a direction different 

 from that which the blood has been stated to 

 pursue. 



7th. That by the operation of transfusion, 

 the blood of one animal may be made to circu- 

 late through the heart and vessels of another, 

 by connecting together the bloodvessels (whe- 

 ther arteries or veins) of the two animals, in 

 such a manner that the course in which the 

 blood is directed by the action of the heart of 

 the animal from which the blood is derived is 

 that of the natural circulation in the animal into 

 which it is introduced. 



8th. The phenomena presented by the circu- 

 lation of the blood in various diseased condi- 

 tions of the heart and bloodvessels may be ad- 

 duced as affording additional illustration of the 

 natural course of the blood, by pointing out the 

 effect of morbid obstructions and other varieties 

 in different parts of the circulatory organs. 



Course of the blood in the foetus before birth. 

 The double circulation just described is the 

 course performed by the blood from the time 

 of birth during the whole of life. 



The circulation of the blood, however, begins 

 at a very early period of foetal life; but the 

 difference in the mode in which respiration is 

 effected in the child so long as it is contained 

 in the uterus, induces a modification in the 

 course of the blood to which we shall now 

 advert. 



There being no inhalation of air into the 

 lungs of the foetus, the blood is sent only in 

 small quantity to these organs, and does not 

 undergo in them any change of properties. A 

 considerable portion of the blood of the foetus 

 passes out of its body through the umbilical 

 cord (Jig. 313, U, u) into the placenta of the 

 uterus. The minutely divided foetal vessels are 

 bathed by the blood of the mother contained in 

 the placental sinuses, and, though no direct 



