642 



CIRCULATION. 



tinct respiratory apparatus; and that, amid the 

 immense varieties of form which the circulatory 

 organs present in different anifnals, the course 

 of the blood bears a more close relation in all 

 to the form of their respiratory apparatus than 

 to any other part of their organization. This 

 general law of the relation between circulation 

 and respiration, satisfactorily established by the 

 extended researches of modern comparative 

 anatomists, receives farther confirmation from 

 many facts connected with the performance of 

 these functions in the adult human body, and 

 is illustrated in a peculiar manner by the re- 

 markable changes which take place in the cir- 

 culatory and respiratory organs of the child 

 before and after birth. 



In treating of the varieties in the course of 

 the blood in different animals, we are at once 

 freed from any embarrassment regarding the 

 order proper to be pursued, by the circumstance 

 that the form of the circulatory organs consti- 

 tutes one of the principal bases upon which the 

 modern classification of animals is founded ; 

 so that, in following the zoological arrange- 

 ment, we take the order best adapted for our 

 present purpose. As our object in giving this 

 sketch is principally to illustrate the structure 

 and functions of the human organs of circula- 

 tion, we shall begin with the consideration of 

 the course of the blood in those animals which 

 most nearly resemble man ; and trace the varie- 

 ties in this function, as far as our knowledge 

 permits, through the descending series of the 

 animal chain. 



1. Course of the blood in warm-blooded 

 animals. In Mammalia and Birds, the form 

 of the organs of circulation and the course of 

 the blood are essentially the same as in Man, 

 for in all of these animals the heart contains 

 four distinct cavities, two auricles and two 

 ventricles, and there is consequently a double 

 circulation and a complete respiration. 



Some considerable varieties in the form of 

 the circulatory organs, which seem to have a 

 relation to peculiarities in habits or mode of 

 life, occur in certain mammiferous animals, 

 such as the Cetacea, Amphibious Qarnivora, 

 the Sloths, Hybernating Animals, &c.; but we 

 shall not at present enter upon the considera- 

 tion of these varieties, because they do not 

 amount to any deviation from the type or 

 general plan of construction of the human 

 organs of circulation, and consequently are not 

 accompanied by any material difference in the 

 course of the blood, but seern rather to have 

 the effect merely of modifying the quantity of 

 blood sent to particular organs, or of influen- 

 cing its velocity and force.* 



In the organs of circulation of the various 

 tribes of Birds, we observe the same remarka- 

 ble uniformity of structure which pervades the 

 rest of their internal organization. 



It may be remarked that, as in Birds a cer- 

 tain respiratory action takes place in the large 

 air-cells distributed over the trunk of the body, 

 and as the pulmonary vessels seem in most 

 birds not to extend to these cells, but to be 



* Sec p. 678. 



confined to the thoracic lungs, the blood con- 

 tained in the small branches of the systemic 

 arteries and veins, ramifying upon the lining 

 membrane of the air-cells, must be made to 

 undergo some respiratory alteration of its com- 

 position ; but we have not as yet obtained the 

 means of judging accurately of the extent to 

 which such a respiratory change may be effect- 

 ed in the vessels of the systemic circulation, 

 nor how far the minute branches of the pulmo- 

 nary vessels may in some instances be pro- 

 longed from the lungs into the air-cells.* 



Very frequent anastomoses take place among 

 the veins of Birds. We may here mention one 

 of these which induces an important modifica- 

 tion in the portal circulation. By means of a 

 communicating branch which passes from the 

 united caudal, hemorrhoidal, and iliac veins to 

 the vena ports, the blood of the viscera of the 

 abdomen and of the posterior part of the body 

 may flow indifferently either into the vena cava 

 inferior or the vena portse, a disposition which 

 may have for its object to prevent congestion of 

 blood in the parts from which these veins pro- 

 ceed .f 



A still more remarkable modification of the 

 venous circulation in Birds was supposed to 

 exist by Professor Jacobson of Copenhagen, 

 consisting in the distribution of branches of the 

 vena cava inferior to the interior of the kidneys 

 and their subdivision in these organs, in the 

 same manner as the vena portae subdivides in 

 the liver. Such veins transmitting venous 

 blood to the kidneys, in (he manner of a vena 

 porte, have been ascertained by Professor 

 Jacobson,| and are admitted by others making 

 subsequent researches, to exist in Reptiles and 

 Fishes; but Nicolai has shewn that the lower 

 veins, described by Jacobson in Birds as venee 

 advehentes of the kidney, do not differ from the 

 other branches of the vena cava, and serve to 

 carry away from these organs, like the superior 

 renal veins of Birds and the renal veins of 

 Quadrupeds, the venous blood derived from 

 the arteries. 



Course of the blood in cold-blooded vertebra- 

 ted animals. Of cold-blooded vertebrated ani- 

 mals, some, as the adult Batrachia, Chelonia, 

 Ophidia, and Sauria, breathe air by means of 

 lungs, while the rest, as the young Batrachia, 

 the Protean, and Siren-like Reptiles and Fishes, 

 are constant inhabitants of water, and breathe 

 the air contained in that medium by means of 

 gills or branchiae. Of the aquatic cold-blooded 

 animals, Fishes breathe by gills only, while 

 the aquatic Reptiles or Amphibia are furnished 

 with lungs as well as gills during the greater 

 part of their aquatic life. 



* See the article Aves, p. 330. 



t It is a remarkable fact that there have been 

 found, between the hemorrhoidal veins in Man and 

 some branches of the vena ports, anastomoses by 

 small branches, which correspond in some respects 

 with the disposition of the veins referred to above. 

 These anastomoses were known to Haller, and are 

 lately described by Retzius. See his Researches in 

 Tiedemann's and Treviranus' Zeitschrift, vol. v. 1. 



| Meckel's Archiv. vol. iii. p. 147. din> Med. 

 and Surg. Journ. vol. xix. p. 78. 



$ Isis, 1826, p. 414. 



