RESPIRATION. 127 



circumstances a negative pressure is established in the thorax in consequence 

 of which the blood is caused to flow with increased rapidity and volume 

 toward the heart. 



Venous Pressure. As the force of the heart-beat 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 ^ 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 twenty to thirty seconds, 

 while for the entire volume of blood to pass through the heart thirty-three 

 pulsations would be required, occupying about thirty-five 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 process by which oxygen is absorbed into the blood 

 and carbonic acid exhaled. The assimilation of the oxygen and the evolution 

 of carbon dioxid 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 a larynx, trachea, and lungs. 



The larynx is composed of firm cartilages, united by ligaments and 

 muscles. Running anteroposteriorly across the upper opening are four 

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

 inferior or true vocal bands 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, f 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. It is com- 

 posed of a series of cartilaginous rings, which extend about two thirds around 



