72 A MANUAL OF VETERINARY PHYSIOLOGY 



pressure in the arteries and veins is high, but in the capillaries 

 of the fist it is nil, owing to their compression. 



Blood-Pressure in the Veins is T \ or yg of that in the large 

 arteries. The greater the distance the veins are from the heart, 

 the greater the pressure, so that the highest pressure is in the 

 peripheral veins and the lowest in the jugular. In a sheep the 

 following values were obtained : 



Jugular vein - - - - o'2 mm. (^ inch). 



Facial vein - - - - 3 „ (| inch). 



Brachial vein - - - - 12 ,, w$ inch). 



Crural vein - - - - 14 „ (£ inch). 



The venous pressure in a dependent part such as the limbs is 

 higher than in a part like the head and neck, where the venous 

 flow is assisted by gravity. During work the venous pressure 

 in the limbs rises, due to the increased ventricular output, con- 

 striction of the abdominal veins, and compression of the veins 



Fig. 29. — Tracing of Arterial Pressure with a Mercury Manometer 



(Foster). 



The smaller curves P, P are the pulse-curves due to the heart-beat. The space 

 from r to r embraces a so-called respiratory undulation. The tracing is 

 taken from a dog, and the irregularities visible in it are those frequently 

 met with in this animal. 



of the limbs. A distended condition of the heart also causes the 

 venous pressure to rise, owing to diminished intake. 



In the large veins just as they enter the heart the pressure is 

 very low. The manometer may even at intervals show a negative 

 pressure. In the anterior vena cava of the dog a negative 

 pressure of —3 mm. (| inch) may be registered. The nega- 

 tive pressure in the veins entering the thorax is due to the 

 fact that the pressure within that cavity is below that of the 

 atmosphere, so that an aspirating effect is produced upon the 

 blood in these veins. It is this aspirating action which renders 

 operations at the root of the neck in the human subject dangerous 

 should air enter the wounded veins. Judging from the writer's 

 observations on the horse, blowing air into the veins causes no 

 discomfort until a considerable amount has been introduced ; 

 even then only sighing respirations are produced. 



Blood-Pressure in the Pulmonary System. — The variations in 

 pressure in the pulmonary system are very much less than those 



