MOVEMENT OF THE BLOOD IN THE ARTERIES. 339 



weight can be withdrawn from the body without materially lowering the 

 blood-pressure. The influence of the respira- 

 tory movements upon the pressure of the blood FlG - 141 - 

 in the vessels is of much interest: after an or- 

 dinary expiration the heart and large vessels in 

 the chest are exposed to a pressure acting 

 through the tissue of the lung, which as Wiindt 

 has pointed out, is equal to the difference be- 

 tween the pressure of the air on the one hand 

 (= 750 mm. of mercury) and the elasticity of 

 the lungs acting in the opposite direction (= 7.5 

 mm. of mercury) on the other ; whilst on the 

 vessels outside the full pressure of the air is ex- 

 erted. The blood-cavities in the chest being 

 thus exposed to less pressure than those of the 

 rest of the body, have a tendency to dilate until 

 restrained by the elasticity of their walls. The 

 venous current is affected to a greater degree Lud wig's stromuhr. 

 than the arterial, since the walls of the veins 



are thinner and less elastic than those of the arteries. In ordinary inspi- 

 ration the elasticity of the lungs rises to 9.5 mm. of mercury, and in extra- 

 ordinary respiration it would even support a column amounting to 30 mm. 

 of mercury in height, and the suction power of the cavities is correspond- 

 ingly increased. In consequence of the descent of the diaphragm this is 

 chiefly exerted upon the veins of the abdominal viscera. In expiration, on 

 the contrary, the pressure on the thoracic vessels being increased, the escape 

 of the blood by the arteries is facilitated, whilst there is a tendency to arrest 

 the flow in the veins; the valves in the latter, however, offer an obstruction 

 to a backward flow, even in extreme expiration, beyond certain limits. The 

 general result of this is, that the pressure of the blood in the veins of the 

 body at large, rises during expiration, and falls during inspiration. In the 

 arterial tracing similar curves occur (complicated of course with those of 

 the cardiac beats), but in the opposite direction, the ascending portion of 

 the curve (corresponding to increase of pressure) occurring chiefly during 

 the period of inspiration, the descending portion of the pressure-curve 

 chiefly occurring during expiration. The rise of blood-pressure in the ar- 

 teries during inspiration depends on the heart becoming filled with more 

 blood from the veins at this period than during expiration, and hence acting 

 more vigorously and at the same time more frequently. But inasmuch as 

 the filling of the cardiac cavities with blood does not take place at the com- 

 mencement of inspiration, nor the diminished supply of blood to them with 

 the commencement of expiration, and since further the diminution of pres- 

 sure within the thoracic cavity in expiration exerts a suctional power upon 

 the contents of the arteries, whilst the increase of pressure during expira- 

 tion tends to drive out the blood, it comes to pass, as was shown by Ein- 

 brodt, 1 that the respiratory phases represented by the dotted line in the 

 following figure (Fig. 145) do not exactly correspond with the variations 

 of pressure, but that the pressure continues to fall for a little while after 

 the commencement of inspiration, and continues to rise for a little while 

 after the commencement of expiration. Curves essentially similar to those 

 observed in normal respiration are seen when the animal is poisoned by 



1 Sitz -ber. d k. Akad. zu Wien, 1859, p. 345. Sre also Funks, Physiolosie, "Band 

 i, 1863, p. 135; Wiindt, Phypiologie, 1873. p. 314; Sanderson, in Handbook for the 

 Physiological Laboratory, 1873, p. 314. 



