1200 



THE RESPIRATORY SYSTEM 



blood circulating in the body. The essential organs in the system are the paired 

 lungs located in the thoracic cavity. Air is carried to and from the lungs by the 

 trachea and bronchi, and these simple transmitting tubes are in turn put into 

 communication with the exterior by the mediation of other organs. The latter 

 are, however, speciallj^ constructed in adaptation to other functions in addition 

 to those relating to respiration: the larynx for the production of the voice, the 

 pharynx and mouth in connection with alimentation, the nasal cavity and external 

 nose functioning in the sense of smell. (For the description of the mouth and 

 pharynx see Section IX; for the olfactory organ see Section VIII.) 



The organs of circulation are always adapted to the form of the respiratory apparatus, and 

 among all higher animals a connection is established between heart and lungs by the pulmonary 

 artery, which carries venous blood to the latter, and by the pulmonary veins, which convey 

 arterial blood from the lungs to the heart, whence the aorta takes it into the general circulation. 



In their origin and development the respiratory organs are closely associated with or 

 differentiated from the beginnings of the digestive apparatus. Thus the processes of the early 

 development of the nasal cavity and mouth are interdependent; the origin of the greater 

 part of the larynx, the trachea and lungs is by ventral outgrowth of the entodermal canal. 



THE NOSE 



The external nose [nasus externus] (fig. 961), shaped like a triangular pyramid, 

 is formed of a bony and cartilaginous framework covered by muscles and the in- 

 tegument of the face externally and lined within by periosteal and perichondral 

 layers overspread by mucous membrane. At the forehead, between the eyes, is 



Fig. 961. — The Left Side of the External Nose, showing its Cartilages, etc. 



Nasal bone 

 Nasal process of the maxilla 



Lateral nasal cartilage 



Nasal septal cartilage 



Lateral crus of greater alar 

 cartilage 



Medial crus of greater alar 

 cartilage 



Sesamoid cartilages 

 Fibrous tissue 

 Lesser alar cartilages 



Cellular tissue forming ala 



the root of the nose [radix nasi], and from this, extending inferiorly and anteriorly, 

 is a rounded ventral border, the dorsum of the nose [dorsum nasi], which may be 

 either straight, convex, or concave, and which ends inferiorly at the apex of the 

 nose [apex nasi]. The superior part of the dorsum is known as the bridge. Inferi- 

 only, overhanging the upper lip, is the base of the nose [basis nasi] which presents 

 two orifices, the nares or nostrils, separated from one another by the inferior mov- 

 able part of the nasal septum [septum mobile nasi]. 



The nostril of man is remarkable on account of its position, facing as it does almost directly 

 downward. It is oval in. form, with the long axis directed antero-posteriorly, or approximately 

 so, in Europeans. The size of the nostril is under the control of muscles (see p. 334) and may 

 be dilated or constricted by their action. 



The sides of the nose slope from the dorsum laterally and posteriorly, and 



