1060 VERTEBRATED ANIMALS 



the spine, where it meets its fellow to form the abdominal aorta, i r 

 which, after giving off branches to the abdominal viscera, is con- 

 tinued as the caudal artery, k, to the extremity of the tail. The- 

 blood is returned from the tail by the caudal vein, I, which i> 

 gradually increased in size by its successive tributaries as it passes 

 towards the abdominal cavity; here it approaches the kidney, in, 

 and sends off a branch which incloses that organ 011 one side, while 

 the main trunk continues its course on the other, receiving tributaries 

 from the kidney as it passes. The venous blood returned from the 

 abdominal viscera, on the other hand, is collected into a trunk, p. 

 known as the portal vein, which distributes it through the substance 

 of the liver, o, as in man ; and after traversing that organ it is dis- 

 charged by numerous fine channels, which converge towards the 

 great abdominal trunk, or vena cava, n, as it passes in close proximity 

 to the liver, onwards to the sinus venosus, q, or rudimentary auricle 

 of the heart. This also receives the jugular vein, r, from the head, 

 which first, however, passes downwards in front of the gill close to 

 its inner edge, and meets a vein t, coming up from the abdomen, 

 after which it turns abruptly in the direction of the heart. Two 

 other abdominal veins, u, meet and pour their blood direct into the 

 sinus venosus ; and into this cavity is also poured the aerated blood 

 returned from the gill by the branchial vein, v, of which only the 

 one on the right side can be distinguished. The lungs may be de- 

 tected in a rudimentary state, even in the very young tadpole, 

 being in that stage a pair of minute tubular sacs, united at the upper- 

 extremities, and lying behind the intestine and close to the spine. 

 They may be best brought into view by immersing the tadpole for a 

 few days in a weak solution of chromic acid, which renders the 

 tissue friable, so that the parts that conceal them may be more 

 readily peeled away. Their gradual enlargement may be traced 

 during the period of the tadpole's transparence ; but they can only 

 be brought into view by dissection when the metamorphosis lias 

 been completed. The following are Mr. Whitney's directions for 

 displaying the circulation in these organs : ' Put the young frog into 

 a wineglass and drop on him a single drop of chloroform. This 

 suffices to extinguish sensibility. Then lay him on the back on a 

 piece of cork and fix him with small pins passed through the web 

 of each foot. Remove the skin of the abdomen with a fine pair of 

 sharp scissors and' forceps. Turn aside the intestines from the left 

 side, and thus expose the left lung, which may now r be seen as a 

 glistening transparent sac containing air-bubbles. With a fine 

 camel-hair pencil the lung may now be turned out, so as to enable 

 the operator to see a large part of it by transmitted light. Unpin 

 the frog and place him on a slip of glass, and then transmit the 

 light through the everted portion of lung. Remember that the lung 

 is- very elastic, and is emptied and collapsed by very slight pressure. 

 Therefore, to succeed with this experiment, the lung should be 

 touched as little as possible, and in the lightest manner, with the 

 brush. If the heart is acting feebly you will see simply a trans- 

 parent sac, shaped according to the quantity of air-bubbles it may 

 happen to contain, but void of red vascularity and circulation. But 



