CIRCULATION IN BLOOD-VESSELS 



81 



Plethysmography. -The arm of a subject (who is to be seated com- 

 fortably) is placed in a Mosso plethysmograph (fig. 67), and this is 

 allowed to rest on a table or in a sling. The junction with the arm 

 is made by a broad rubber band. The interior of the plethysmograph 

 is connected by rubber tubing with a recording tambour or a 

 bellows- or piston-recorder ; the whole is to be airtight. The lever 

 of the recorder registers respiratory and cardiac movements upon the 

 smoked surface, since these movements produce changes in general 

 arterial pressure, and thus in the amount of blood driven into the arm. 

 Compress the brachial vein above the elbow ; the swelling of the 

 arm due to retention of blood is at once shown. 



Plethysmographs for the kidney (see fig. 68), spleen, and other organs may 

 be also studied. The principle of their 

 action is the same as that used for the 

 arm, but the form is adapted to each 

 particular organ. The gap (g) in the 

 instrument through which the vessels 

 and duct pass is made airtight by 

 vaseline. 



Study of blood pressure, and the 

 effects of nerves and drugs upon the 

 heart and arteries in animals. Either an 

 anaesthetised animal may be employed as 

 the subject of a demonstration on blood- 

 pressure, or a " Sherrington " preparation 

 (p. 75) can be used by the student. A 

 cannula (fig. 64, e) is inserted into one of 

 the carotids towards the thorax and tied 

 securely in. The artery is previously 

 ligatured above and clamped below the 

 place of insertion. The cannula is con- 

 nected by a tube containing solution 

 of sodium bicarbonate with the proximal 

 limb of a mercurial manometer ; a 

 lateral tube affords communication with 

 a reservoir of the same solution, which 

 is under pressure. By this pressure the 

 mercury is forced up in the distal limb 

 of the manometer, so that the recording 

 style is about 50 mm. above the abscissa 

 or zero line. The clamp on the artery 

 is then removed, and a record of the 

 arterial pressure taken, a time record 

 (in 10-second intervals) being also in- 

 scribed. It is useful to record the 

 respiratory movements at the same 

 time : this is conveniently done either 

 by tambours or by attaching a thread 

 by one end to the skin of the epigastric 

 region and causing the other end to 

 activate a lever. A blunt metal cannula, 

 which fits the nozzle of a hypodermic 

 syringe, is tied into} one of the jugular 

 veins towards the thorax : this is for the purpose of giving intravenous in- 

 jections of drugs and extracts. A kidney or the spleen or a loop of intestine, 



6 



e 



FIG. 69. LUDWIG'S STROMUHR. a, a', CAN- 



NULAS FOR TYING INTO CUT ARTERY ; 



6, BLOCK ON WHICH THE PART c ROTATES 



ABOUND THE AXIS, d \ e, KESERVOIR CON- 

 TAINING OIL ; e 1 , RESERVOIR CONTAINING 

 DEPIBRINATED BLOOD ; /, APERTURE FOR 

 FILLING RESERVOIRS, CLOSED BY CORK, g. 



