CIRCULATION. 



ries ; by the branchial veins it gains the lower 

 vessel. This vessel may be regarded as the 

 systemic artery, and sends the arterial blood, 

 by the numerous anastomosing branches, up- 

 wards across the intestine, and through the 

 other parts into the upper vessel. The upper 

 vessel communicates also with the lower ante- 

 riorly by the lateral dilatations named auricles, 

 which are supposed to furnish some blood to 

 the upper vessel. A part of the blood at the 

 anterior extremity of the lower vessel is said to 

 be propelled into the two subordinate vessels 

 placed along the sides of the nervous cord. 

 In this course which the blood is stated to 

 follow, it does not appear to be known whether 

 its motion is of a regular progressive kind or 

 only undulatory. 



Leech. In the leech the principal and most 

 highly contractile longitudinal vessels are placed 

 one on each side (Jig. 324, a, a), and there are 

 also two lesser longitu- 

 Fig. 324. dinal vessels, one supe- 



rior and the other inferior 

 (a*), all which commu- 

 nicate freely together by 

 small cross branches along 

 the whole body (c). It 

 is remarkable that the lower 

 median vessel (a*) incloses 

 the ganglionic nervous 

 cord, so as to bathe it with 

 blood. Both pulmonary 

 arteries and veins are 

 branches of the lateral ves- 

 sels ; a capillary network 

 between them distributing 

 the blood minutely over 

 the pulmonary sacs or 

 vesicles. The pulmonary 

 veins form very remarkable 

 dilated and coiled por- 

 tions, which seem to be 

 endowed with a high de- 

 gree of contractility. Ac- 

 cording to J. Miiller, for 

 a certain number of pul- 

 sations, the middle and the 

 lateral vessel of one side 

 contract together, and pro- 

 pel the blood into the 

 lateral vessel on the other 

 side, and then the order 

 is reversed, and the middle 

 vessel acts along with 

 the lateral vessel of the 

 other side, so that one lateral vessel is always 

 dilated while the median and opposite lateral 

 ones are contracted, and vice versa. According 

 to some there is thus only an alternate motion 

 of the blood from one side to the other, while 

 others believe that there is at the same time a 

 gradual progressive motion of the blood for- 

 wards in the upper vessel and backwards in 

 the lower one.* 



The course of the blood in the principal 



* See a full account of most of the opinions of 

 observers on this subject, as well as original obser- 

 vations by Rudolf Wagner, in the Isis for 1832, 

 p. 643. 



Erpobdella or Leech. 



parts of the circulatory organs is nearly the 

 same in the rest of the Articulata, viz. Crustacea, 

 Arachnida, and Insects, as in Annelida. In all 

 of them the central propelling organ, whether in 

 the form of a heart or consisting only of a dilated 

 arterial vessel, such as the dorsal vessel of in- 

 sects, is situated on the upper surface of the 

 animal, above the alimentary canal, while the 

 returning vessels are situated on the lower sur- 

 face of the body, on each side of the nervous 

 ganglionic cord. The respiratory circulation, 

 when occurring in a distinct set of vessels, 

 forms a part of the venous system, and the 

 heart, which has no auricle, is systemic or 

 aortic. 



Insects. All perfect Insects, whether inha- 

 bitants of air or water, breathe air alone. In 

 these animals there is not a separate and dis- 

 tinct respiratory organ in one part of the body 

 only, but the atmospheric air is carried by 

 minute elastic and tough tubes ramified to an 

 infinite degree of minuteness into every part of 

 their body. 



The dorsal vessel of insects forms a long and 

 wide contractile artery, larger in general behind 

 than before, in which the contractions begin at 

 the posterior extremity, and proceed gradually 

 forwards with an undulatory motion. In the 

 greater number of perfect insects, we are not 

 acquainted with any other vessels or passages 

 in the body, through which the blood moves, 

 and this fluid seems in these insects to oscillate 

 backwards and forwards in the dorsal vessel 

 alone. This state of the circulation in insects, 

 according to the ingenious views of Cuvier, is 

 related to the distribution of the respiratory 

 organ over the whole body, in consequence of 

 which the air is brought in contact with the 

 more perfect blood contained in the dorsal 

 vessel, and the nutritious fluids supposed to 

 pervade interstitially the rest of the body. The 

 recent discovery by Carus of a continuous cir- 

 culation of the blood through arteries and veins 

 in a few of the perfect insects, and more espe- 

 cially in some larvae, must modify the above 

 views, which, ingenious as they must appear to 

 all, do not account so satisfactorily for the ab- 

 sence of a systemic as for the want of a pulmo- 

 nary circulation. The circulation of the blood 

 of Insects may be most easily seen in the 

 aquatic larvae of Neuropterous Insects, as the 

 Agrion, Ephemera, Semblis, and Libellula,* in 

 which it was first discovered. 



In these larvae it may be described generally 

 as follows. The dorsal vessel (/g.325, H) is 

 connected anteriorly and posteriorly by several 

 branches with the inferior or returning vessels 

 (v, v\ which, running along the whole body, 

 receive the blood from the anterior extremity, 

 and carry it into the posterior extremity of the 

 dorsal vessel. The antennae and first joint of 

 the legs, as well as the fin-shaped caudal pro- 

 cesses, receive each a loop of vessel from the 

 abdominal current; and from the motion of the 

 globules in these transparent parts, the circula- 

 tion can be more easily seen in them than in 



* We have ourselves seen the circulation in the 

 larvae of two Neuropterous Insects. 



2 u 2 



