STKUCTUEE OF PULMONIBRANCHI^E. 377 



it gives off two pairs of vessels at its sides. The first and second pairs 

 of these efferent vessels, covered hy the thick peritoneal lining of the 

 abdomen, send the blood in a diagonal direction backwards to the first 

 pair of abdominal branchiae. The first pair of these vessels originate 

 close to the folds of the diaphragm. They pass backwards and ontwards 

 into the abdomen, and are joined in their course by numerous small 

 vessels from the sides of the segments, and immediately anterior to the 

 first pair of abdominal branchiae are each divided into two branches, 

 which are again divided and subdivided into a multitude of anasto- 

 mosing vessels before they are distributed on the branchiae. These 

 branchiae likewise receive the second pair of efferent vessels, which, like 

 the first, pass diagonally backwards from the fibrous structure to the 

 inner side of the branchiae, on approaching which they are divided like 

 the other pair into two branches, which are subdivided, and anastomose 

 with the divisions of the first pair. The whole form a most intricate 

 web of anastomosing pulmonic capillary vessels before they are distri- 

 buted on the anterior part of the branchiae. We have thus a complete 

 distribution of the blood to the pulmonibranchiae in the anterior part 

 of the abdomen. There is a similar but less perfect distribution in the 

 posterior. 



(971.) Professor Miiller* has accurately described the pulmoni- 

 branchiae as formed of a multitude of closely- approximated, thin, double 

 lamellae, which communicate, by a small orifice in each, with the ex- 

 ternal air admitted into a common cavity through the spiracle on the 

 surface of the body. The blood, distributed through these lamellae, is 

 brought into contact with the air in their interior through their mem- 

 branous structure. The minute anatomy of these lamellae, and the 

 manner in which they are permeated by the blood, afford some points of 

 interest. Each side of these double lamellae is formed of an exceedingly 

 delicate and apparently structureless double membrane, which includes 

 within it a parenchymatous tissue formed of single vesicles or cells. 

 The convex margin of each lamella is bounded by a delicate but distinct 

 vessel, which seems to form the means of intercommunication with 

 the anastomosing network of vessels distributed over the branchiae, 

 since the delicate evanescent vessels traced into the lamellae are derived 

 from those which bound their convex margin. 



(972.) At the posterior part of the inner side of the branchiae, 

 where the lamellae are covered by the thick membrane and peritoneum 

 that lines the common cavity of the branchiae, there are several small 

 orifices, the commencement of vessels which afterwards, when collected 

 together, form the larger channels that convey back the blood to the 

 heart. These vessels form delicate trunks or sinuses, which pass around 

 the sides of the body in the posterior part of each segment, and, be- 

 coming gradually enlarged by communicating with other vessels in their 

 * Meckel's Archiv, 1828. 



