RESPIRATION. 69 



diameters, and the pressure on its contents at once diminishes. Under 

 these circumstances a suction force is exerted upon the great venous trunks, 

 which causes the blood to flow with increased rapidity and volume toward 

 the heart. 



Venous pressure. As the force of the heart is nearly expended in 

 driving the blood through the capillaries, the pressure in the venous system 

 is not very marked, not amounting in the jugular vein of a dog to more 

 than Jj that of the carotid artery. 



The time required for a complete circulation of the blood throughout the 

 vascular system has been estimated to be from 20 to 30 seconds, while for 

 the entire mass of blood to pass through the heart 58 pulsations would be 

 required, occupying 48 seconds. 



The Forces keeping the blood in circulation are 



1. Action of the heart. 



2. Elasticity of the arteries. 



3. Capillary force. 



4. Contraction of the voluntary muscles upon the veins. 



5. Respiratory movements. 



RESPIRATION. 



Respiration is the function by which oxygen is absorbed into the 

 blood and carbonic acid exhaled. The appropriation of the oxygen and 

 the evolution of carbonic acid takes place in the tissues as a part of the 

 general nutritive process ; the blood and respiratory apparatus constituting 

 the media by means of which the interchange of gases is accomplished. 



The Respiratory Apparatus consists of the larynx, trachea and lungs. 



The Larynx is composed of firm cartilages, united together by liga- 

 ments and muscles; running antero-posteriorly across the upper opening are 

 four ligamentous bands, the two superior, or false vocal cords, and the two 

 inferior, or true vocal cords, formed by folds of the mucous membrane. 

 They are attached anteriorly to the thyroid cartilages and posteriorly to the 

 arytenoid cartilages and are capable of being separated by the contraction 

 of the posterior crico-arytenoid muscles, so as to admit the passage of air 

 into and from the lungs. 



The Trachea is a tube from four to five inches in length, three-quarters 

 of an inch in diameter, extending from the cricoid cartilage of the larynx 

 to the third dorsal vertebra, where it divides into the right and left bronchi. 



