54 HUMAN PHYSIOLOGY. 



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 processs ; 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. 

 It is composed of a series of cartilaginous rings, which extend about two- 

 thirds around its circumference, the posterior third being occupied by 

 fibrous tissue and non-striated muscular fibres which are capable of dimin- 

 ishing its calibre. 



The trachea is covered externally by a tough, fibre-elastic membrane, 

 and internally by mucous membrane, lined by columnar ciliated epithelial 

 cells. The cilia are always waving from within outward. When the two 

 bronchi enter the lungs they divide and subdivide into numerous and 

 smaller branch'es, which penetrate the lung in every direction until they 

 finally terminate in i\it pulmonary lobules. 



As the bronchial tubes become smaller their walls become thinner ; the 

 cartilaginous rings disappear, but are replaced by irregular angular plates 



