214 



OP THE BLOOD AXD CIKCULA-TTON". 



apparent ; single streams are often only recognizable by tlieir 

 bounding parietes ; comprehended within two dark lines, 

 these vessels are usually filled with the liquor sanguinis 

 alone ; it is at intervals only that a blood-corpuscle, more 

 rarely a lymph-corpuscle, from some neighbouring and larger 

 streamlet, detaches itself and makes its way into the canal, 

 which till now had appeared empty ; one corpuscle entering 

 in this way is frequently followed by several others in pretty 

 rapid succession, and then, or without anything of the kind 

 occurring, the vessel for a long time circulates nothing but 

 the limpid plasma. Whether there are any vessels or not 

 that never circulate aught but plasma, refusing, by reason of 

 the smallness of their diameters, at all times to admit the 

 blood-corpuscles, is doubtful. 



[ 375. Such is the peripheral systemic circulation in every 

 tissue susceptible of special examination. In the peripheral 

 vessels of every part yet examined, the separation into the 

 quicker stream of blood-corpuscles in the centre, and of the 

 slower one of liquor sanguinis in the circumference above in- 

 dicated, has been observed. But the circulation of the respi- 

 ratory apparatus, whe- 

 ther lungs or gills, offers 

 a most remarkable ex- 

 ception to this rule, so 

 uniform in reference to 

 the circulation at large. 

 The capillaries of the 

 respiratory organ are 

 filled with blood gene- 

 rally, i. e. liquor san- 

 guinis, with its super- 

 added blood and lymph- 

 corpuscles, to their 



Fig. 231. One of the pulmonary islets very walls (figs. 230 and 

 bounded by capillaries on three sides, by a 231.) It is only in the 

 larger venous branch on the fourth side, larger capillary vessels 

 a, b, c are lymph-globules mingled with the t j iat a tllin stra tum of 

 blood-globules. The object is magnified i f h 



about 300 times. plasma is 



contact with theparietes, 



which are much more delicate than those of the systemic 

 circulation, and not, like them, formed of a series of dark 



