EFFECT OF HEAT AND COLD OX PULMONARY CAPILLARIES. 333 



The action of the drugs on the puhnonary capillaries in the 

 frog may be observed directly by means of the microscope. A 

 frog is ciirarised, and a glass cannula, to which a short piece of 

 india-rubber tubir.g is attached, is tied into its larynx. An 

 incision is tlien made into the side of the frog a little belo\Y the 

 arm, care being taken not to injure the lung in dividing the 

 thoracic wall ; and, the lungs being inflated through the 

 cannula, one of them protrudes througli the opening. Tlie 

 piece of india-rubljer at the end of tlic cannula must then be 

 clamped, so as to prevent the air from escaping and the lung 

 from collapsing (Fig. 14G). The whole frog is then placed en a 



Fig. 14G. — Fonii of Tube for insertion into the Larjnx of the Frog. 



h a 



10) 



It is made by drawing out a piece of tubing to the size marked a, heating the 

 end and pressing it against a piece of metal, so that it assumes the shape h, 

 c is a piece of india-rubber tubing, which must be closed, cither by a ligature, 

 as shown in the drawing, or, what is still better, by a clip, so as to prevent the 

 escape of air from the lung. 



glass plate, which is fixed with a clip to the stage of the 

 microscope. The lung is brought under the objective, and 

 supported, if necessary, by a cork ring fixed to the glass plate 

 with sealing-w^ax. Any changes in the calibre of the pulmo- 

 nary vessels are then readily observed by means of a micrometer 

 placed in the eye-piece. 



Four years ago I attempted to ascertain by this method 

 (Fig. 147) the effect of sudden changes from heat to cold on the 

 lungs ; and 1 have found that, if a stream of warm moist air be 

 first directed on the lung, and immediately afterwards a stream of 

 cold moist air, the capillaries sometimes contract as much as 

 one-third of their diameter under the influence of the cold. 



Dr. Sharpey informs me that the lung of the toad, unlike 

 that of the frog, does not collapse even when no obstruction is 

 offered to the escape of air ; and therefore, if toads be used, 

 the india-rubber tube and clamp on the cannula are unnecessary. 



