SANGUISUGA. 



lateral vessels for the necessary purpose of undergoing respi- 

 ration. 



The two lateral vessels are appropriated to the supply of 

 the respiratory system, and in them the blood moves in a 

 circle quite independent of that formed by the dorsal artery 

 and ventral vein, although they all communicate freely by 

 means of cross-branches, those passing from the lateral vessels 

 to the dorsal being called by M. Duges dor so-lateral (Annales 

 des Sciences Nat., Vol. XV.), while those which join the lat- 

 eral trunks to the ventral canal are the latero-abdominal 

 branches of that observer. The movement of the blood in 

 the lateral or respiratory system of vessels is quite distinct 

 from that which is accomplished in the dorso- ventral, or sys- 

 temic trunks ; sometimes it passes down the vessel from the 

 head towards the tail, and in an opposite direction on the 

 other side of the body, but in a short time the movement of 

 the currents will be seen to become completely reversed, so 

 that an undulatory motion, rather than a complete circulation, 

 is kept up. By this action of the lateral canals, the blood is 

 made perpetually to pass and repass the respiratory sacculi, 

 and opposite to each of these, branches are given off which 

 form so many independent vascular circles, representing very 

 closely the minor or pulmonary circulation of the higher ani- 

 mals. 



On examining attentively one of the respiratory pouches, 

 its membranous walls are seen to be covered with very fine 

 vascular ramifications, derived from two sources ; the latero- 

 abdominal vessel gives off a branch, which is distributed upon 

 the respiratry sacculis, and there is another very flexuous vas- 

 cular loop derived from the lateral vessel, which terminates 

 by ramifying upon the vesicle in a similar manner. The walls 

 of the loop are extremely thick and highly irritable, but on 

 tearing it across, the internal cavity or canal by which it is 

 perforated is seen to be of comparatively small diameter, so 

 that it is not surprising that, although such appendages to the 

 respiratory sacs were detected and well delineated by former 

 anatomists, their nature was unknown, and they were supposed 

 to be glandular bodies appropriated to some undiscovered use. 

 From the arrangement above described, it is evident that 

 small circular currents of blood exist, which are independent, 

 to a certain extent, of the general circulation, since opposite 

 to each membranous bag a portion of the fluid contained in 



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