VIII. RESPIRATION. 



A STUDY of the phenomena of animal life teaches us that a supply of 

 oxygen and an elimination of carbon dioxide are essential to existence. Oxy- 

 gen is indispensable to life ; carbon dioxide is inimical to life. One serves for 

 the disintegration of complex molecules whereby energy is evolved, while the 

 other is one of the main effete products of this dissociation. We therefore find 

 an intimate relationship between the ingress of the one and the egress of the 

 other. During the entire life of the individual there is this continual inter- 

 change, which we term respiration. This term embraces two acts which, while 

 different, are nevertheless co-operative first, the interchange of O and CO 2 ; 

 second, the movements of certain parts of the body, having for their object the 

 inflow and outflow of air to and from the lungs. The former, properly speak- 

 ing, is respiration ; the latter, movements of respiration. 



Respiration is spoken of as internal and as external respiration. In the 

 very lowest forms of life the interchange of gases takes place directly between 

 the various parts of the organism and the air or the water in which the organ- 

 ism lives ; but in higher beings a circulating fluid becomes a means of 

 exchange between the bodily structures and the surrounding medium, so that 

 in these beings there is first an interchange between the air or the water in 

 which the animal lives and the circulating medium, and subsequently an inter- 

 change between the circulating medium and the tissues. Therefore in the most 

 primitive forms of life respiration is a single process, while in higher organ- 

 isms it is a dual process, or one consisting of two stages, the first being the 

 interchange between the atmosphere or the water surrounding the body and 

 the circulating medium, and the second between the circulating medium and 

 the bodily structures. In man, external respiration is the interchange taking 

 place between the blood and the gases in the lungs and between the blood and 

 the air through the skin ; while internal respiration is the interchange between 

 the blood and the tissues. In external respiration O is absorbed and CO 2 is 

 given off by the blood ; in internal respiration the blood absorbs CO 2 and 

 gives off O. 



A. THE RESPIRATORY MECHANISM IN MAN. 



The respiratory apparatus in man consists (1) of the lungs and the air- 

 passages leading to them, the thorax and the muscular mechanisms by means 

 of which the lungs are inflated and emptied, and the nervous mechanisms con- 

 nected therewith ; and (2) the skin, which, however, plays a subsidiary part in 

 man, and need not here be considered. 



503 



