BY DR. BUllDON-SA^ 7 DERSON. 289 



which the upper, guarded by a screw-clamp, communicates 

 with a vessel at a higher level, the lower conveys the water 

 away as fast as it is supplied. The excellency of this method 

 lies in the fact that the animal can be kept under observation, 

 without the use of any narcotizing drug, for a long time in a 

 perfectly natural condition. The frog is used both in the larval 

 and adult state. To observe the circulation in the tail of the 

 tadpole, the animal is placed in a moderately strong solution 

 of curare, care being taken to remove it before it is completely 

 paralyzed the moment, in short, that its motions become 

 sluggish. It is also possible to secure it, without the aid of 

 curare, in a holder of construction similar to that of the in- 

 strument I have just described a method which has this great 

 advantage, that the animal is in a more normal condition ; for 

 even when curare is given with the greatest care, the action 

 of the heart is weakened by it. For most purposes the adult 

 frog is more useful than the tadpole, particularly when it is 

 desired to observe not merely the circulation as it is, but to 

 witness the modifications which the phenomena undergo under 

 the influence of conditions acting on the bloodvessels through 

 the nervous system. 



There are three transparent parts of the frog the mesen- 

 tery, the web, and the tongue each of which has its special 

 advantages for the purposes of study. For a first view of the 

 relation between arteries, capillaries, veins, and lymphatics, the 

 mesentery is superior to either of the other two. The frog 

 must be placed under the influence of curare, the dose of 

 which, for the ordinary specimens of rana temporaria, is about 

 ^oo^ 1 of a grain. The solution of curare is prepared by 

 weighing out five milligrammes of the substance, and rubbing 

 it up in a glass mortar with a little alcohol. The proper 

 quantity of water that is, sufficient to make up ten cubic 

 centimetres is then added, and a straw-colored, nearly limpid 

 liquid is obtained; a single drop of which is a sufficient dose. 

 It is injected under the skin of the back with an ordinary 

 subcutaneous syringe, and answers best when the effect does 

 not manifest itself for some time after the injection. The 

 most convenient apparatus for the purpose of exposing the 

 mesentery is that shown in fig. 217. The manipulation is 

 fully described in Chapter VII. It is always desirable to 

 commence the examination with a low power. It is then seen 

 that the arteries are smaller than the veins, the latter exceed- 

 ing the former in diameter by about a sixth ; that the arterial 

 stream is quicker than the venous ; that it is accelerated 

 appreciably at each beat of the heart ; and that in every 

 artery a space can be distinguished within the outline of the 

 vessel, which is entirely free from corpuscles. The arterial 

 stream, indeed, is so quick that the forms of the corpuscles 



