MEASUREMENT OF THE VELOCITY OF THE BLOOD-CURRENT. 171 



pressure in the greater circulation, has the same effect in the lesser circulation 

 because more blood flows into the right heart. The vessels of the lesser circulation 

 are exceedingly elastic and their tonicity is slight; hence impermeability even of 

 large pulmonary branches is readily compensated for. 



Forcible contraction of the abdominal muscles (straining) causes at first a 

 marked increase in the flow of blood from the pulmonary veins, which, however, 

 gradually ceases, because the blood finds difficulty in entering the pulmonary 

 vessels. When the abdomen is relaxed, the blood again enters the pulmonary 

 vessels in large quantities. 



Noteworthy in this connection are the experiments of Severini, who found 

 that the flow of blood through the pulmonary vessels is freer and more rapid when 

 the lungs are filled with air rich in carbon dioxid, than with air containing a larger 

 percentage of oxygen. He believes that these gases affect the vascular ganglia 

 in the lesser circulation that control the size of the vessels. 



According to Morel, electrical and mechanical stimulation of the abdominal 

 organs causes a considerable increase of the blood-pressure in the pulmonary 

 artery (dog). According to v. Basch, increase of blood-pressure in the capillaries 

 of the lungs produces greater rigidity and, therefore, diminished elasticity of the 

 alveolar walls. 



Pathological. The pressure in the pulmonary area is increased in man in 

 connection with many morbid disturbances of the circulation and always produces 

 accentuation of the second pulmonic sound, which is such an important pathogno- 

 monic sign. It also causes an increase in size and an earlier appearance of the 

 corresponding elevation in the apex-beat curve. But little has been determined 

 with regard to the effect of physiological conditions; temporary suspension of 

 breathing is said always to be followed by an increase in pressure. The influ- 

 ence of the vasomotor nerves on the vessels of the lesser circulation is not so 

 great as that upon those of the greater circulation. Influences that cause a 

 rise or a fall in the blood-pressure in the greater circulation through the agency 

 of the vasomotor or vasodilator nerves have no effect whatever on the pressure 

 in the lesser circulation. Plethora of the pulmonary capillaries is followed by 

 enlargement of the lungs, with more complete distention of the alveoli. The 

 causes may be a diminished flow from the pulmonary veins or disturbances in the 

 left heart. The development of pulmonary edema is discussed on p. 224. 



MEASUREMENT OF THE VELOCITY OF THE BLOOD-CURRENT. 



The following instruments are used for determining the velocity of the blood- 

 current in the vessels : 



i. Alfred Wilhelm Volkmann's hemodromometer measures directly the progress 

 of the blood-column through a glass tube in a blood-vessel. 



A glass tube shaped like a hairpin, 130 cm. long and 2 or 3 mm. wide and 

 mounted on a scale (Fig. 68, A), is fastened to a metallic basal piece (B) in such 

 a manner that each limb passes to a stopcock perforated all the way through 

 in one direction and halfway through in the other. The basal piece is perforated 

 lengthwise and the two extremities are provided with short cannulae (c c), 

 which are tied into the two ends of the divided blood-vessel. The entire 

 apparatus is next filled with a 0.6 per cent, sodium-chlorid solution. The stop- 

 cocks, which are provided with an arrangement of cogs so that they always turn 

 together, are first placed as shown in Fig. I : the blood then simply flows length- 

 wise through the basal piece; that is, in the same straight direction as the artery. 

 If at a given moment the stopcocks are turned as shown in Fig. 68, II, the blood 

 is forced to flow through the longer channel represented by the glass tube. The 

 blood will be seen pushing the paler column of water before it and the instant 

 should be noted at which it reaches the extremity of the limb of the tube. The 

 length of the tube being known and the time occupied by the blood in passing 

 through it being determined, the velocity for the unit of time and the unit of length 

 of the course is readily obtained. 



Volkmann found the velocity of the current in the carotid of the dog 

 to be between 205 and 357 mm. ; in the carotid of the horse, 306; in the 

 facial of the horse, 232 ; and in the metatarsal artery, 56 mm. 



The observation occupies only a few seconds. The tube is narrower than the 



