the lack of a stomach may reflect the specialized diet. Lack 

 of a spleen is peculiar to the cyclostome. 



Although the abdominal viscera do not sharply separate 

 the agnath from the gnathostome, or define the several sub- 

 groups of the latter, they do contribute to the image of the 

 primitive vertebrate. This ancestral form had a complex of 

 organs rather similar to that of the lamprey but had in ad- 

 dition a stomach and pancreas, and perhaps a spleen. 



THE RESPIRATORY SYSTEM 



Respiration is usually defined as breathing, an act which 

 involves the movement of air into and out of the lungs or 

 the movement of water past gills, as in the fish. Respiration 

 as a basic biological function is a much more sophisticated 

 concept. For our purposes, respiratory systems involve organs 

 specialized for the exchange of oxygen and carbon dioxide 

 between the organism and the environment. Such a system 

 includes a vascular contribution, since the exchange is be- 

 tween the environment and the blood. 



Lungs 



Mammals The lungs are bilateral lobulate structures, ly- 

 ing in the pleural cavities of the chest. The lungs are formed 

 largely from the terminal sacculations of branching tubules 

 stemming from the midline trachea, which opens into the 

 pharynx through a T-shaped or ovoid glottis. Anterior to 

 the glottis is the raised epiglottis, which folds back to help 

 close the glottis. The epiglottis is part of the complex 

 pharyngeal apparatus, the larynx, formed from cartilages 

 of the branchial arches. The cartilages involved in this ap- 

 paratus are the epiglottis, thyroid, cricoid, and arytenoid. 

 The larynx contains the vocal cords, which function in 

 speech. 



Distally the trachea branches into two tubes, the bronchi. 

 From these, second order bronchi branch irregularly into 

 the lobes of each lung. The first few orders of bronchi are, 

 like the trachea, supported by cartilaginous semirings. 

 These successive branchings end with the lobular and the 

 respiratory bronchioles. The latter divide into alveolar 

 ducts opening into a large number of alveoli or into alveolar 

 sacs, surrounded in turn by alveoli. The alveolus is the ter- 

 minal division of this branching system and the site of gas 

 exchange. 



The pulmonary arteries and veins follow the branching 

 of the air ducts. The pulmonary arteries stem directly from 

 the right ventricle of the heart, while the pulmonary veins 

 return to the left atrium. 



The cavities in which the lungs lie are separated from 

 that of the heart by the pericardial sac, and from the ab- 

 dominal cavity by the diaphragm. The diaphragm is mus- 

 cular and its contractions, along with those of the inter- 

 costal muscle, increase the thoracic volume. The lungs fol- 

 low the volume changes of the thoracic cavity passively, so 

 that air moves in and out of the air passages and body. 



The air moving in and out of the body utilizes the nasal 

 passages. These are separated from the mouth cavity by the 

 secondary, or false, palate. In the nasal passages the air is 

 sampled for odors by the olfactory sense. The nasal passages, 

 with their intricate turbinals, condition the air in terms of 

 temperature and moisture content for its entrance into the 

 lungs. 



Among mammals the lungs have a varying number of 

 lobes; the right is usually more subdivided than the left. 

 The left may be undivided, or both lungs may be undivided 

 as in the whales. In whales the lungs extend back dorsally 

 for nearly half the body cavity length; the diaphragm is 

 powerful and obliquely oriented. The lungs function for 

 hydrostatic purposes as well as respiration. In the whale 

 there is a great development of arborized tubules supported 

 by cartilaginous rings, but a reduced amount of alveolar 

 tissue. 



EMBRYOLOGICAL DEVELOPMENT The lungs appear first as a 

 ventral diverticulum from the posterior floor of the pharynx, 

 the laryngotracheal groove (Figure 9-6 D). This diverticu- 

 lum extends back below the esophagus and branches to right 

 and left in the tissue below the esophagus. The pair of lung 

 buds thus formed bulge into the body cavity to either side, 

 and, as development progresses, these bulges extend back- 

 ward and outward separate from the esophagus (Figure 

 9-29), 



The cutting off of the pulmonary spaces from the rest of 

 the body cavity involves folds which grow inward from the 

 body wall. The pleuropericardial folds form between the 

 lungs and the pericardial space, while the pleuroperitoneal 

 folds cut off the pleural cavity from the abdominal cavity. 

 These folds join the transverse septum, which is associated 

 with the anterior face of the liver, and the dorsal septum of 

 the gut, the mediastinum. The latter supports the dorsal 

 aorta, esophagus, and cava! veins. The diaphragm is formed 

 from the transverse septum and pleuroperitoneal folds at- 

 tached medially to the mediastinum, and it is invaded 

 laterally by its musculature. 



Reptiles The lungs of reptiles and amphibians are similar, 

 except that those of the former tend to be more complexly 

 sacculated. In Sphenodon or the lizard, the right and left 

 bronchi open into large central cavities surrounded by 

 spongy alveolar tissue. The right lung is usually the larger. 

 In turtles (Figure 9-10), the alveolar tissue may be very ex- 

 tensive with channels leading into it from the restricted 

 central cavity. Alveolar tissue may enclose regularly arranged 

 chambers with restricted openings into the central cavity. In 

 the alligator the central cavity is little more than an exten- 

 sion of the bronchus into the alveolar tissue. 



The lung of the reptile is supported by a mesentery from 

 the dorsal mesentery. In most reptiles the lungs extend back 

 into the abdominal cavity. In the alligator the somatic peri- 

 toneal membrane enclosing the visceral mass is muscular 

 and separated from the body-wall musculature. From its 



280 • THE VISCERA 



