The bronchus and pulmonary artery in this lung type maintain a close relationship throughout .
The pulmonary vein , however , without the limiting supportive tissue septa as in type 1 , , follows a more direct path to the hilum and does not maintain this close relationship ( figs. 8 , 22 ) .
Another marked difference is noted here .
The pulmonary artery , in addition to supplying the distal portion of the respiratory bronchiole , the alveolar duct , and the alveoli , continues on and directly supplies the thin pleura ( fig. 8 ) .
The bronchial artery , except for a small number of short branches in the hilum , contributes none of the pleural blood supply .
It does , as in type 1 , , supply the hilar lymph nodes , the pulmonary artery , the pulmonary vein , the bronchi , and the bronchioles -- terminating in a common capillary bed with the pulmonary artery at the level of the respiratory bronchiole .
No bronchial artery-pulmonary artery anastomoses were noted in this group .
Lung type 3 ( ( fig. 3 ) is to some degree a composite of types 1 , and 2 .
It is characterized by the presence of incompletely developed secondary lobules ; ;
well defined , but haphazardly arranged , interlobular septa and a thick , remarkably vascular pleura ( fig. 9 ) .
The most distal airways are similar to those found in type 1 , , being composed of numerous , apparently true terminal bronchioles and occasional , poorly developed respiratory bronchioles ( figs. 14 , 15 ) .
In this instance , because of incomplete septation , the secondary lobule does not constitute in itself what appears to be a small individual lung as in type 1 .
Air-drifts from one area to another are , therefore , conceivable .
Distally the bronchus is situated between a pulmonary artery on one side and a pulmonary vein on the other , as in type 1 ( ( fig. 24 ) .
This relationship , however , is not maintained centrally .
Here the pulmonary vein , as in type 2 , , is noted to draw away from the bronchus , and to follow a more direct , independent course to the hilum ( figs. 23 , 24 ) .
The bronchial artery in its course and distribution differs somewhat from that found in other mammals .
As seen in types 1 , and 2 , , it supplies the hilar lymph nodes , vasa vasorum to the pulmonary artery and vein , the bronchi and the terminal bronchioles .
As in type 1 , , it provides arterial blood to the interlobular septa , and an extremely rich anastomotic pleural supply is seen ( figs. 9 , 10 ) .
This pleural supply is derived both from hilar and interlobular bronchial artery branches .
Such a dual derivation was strikingly demonstrated during the injection process where initial filling would be noted to occur in several isolated pleural vessels at once .
Some of these were obviously filling from interlobular branches of the bronchial arteries while others were filling from direct hilar branches following along the pleural surface .
With completion of filling , net-like anastomoses were noted to be present between these separately derived branches .
An unusual increase in the number of bronchial arteries present within the substance of the lung was noted .
This was accounted for primarily by the presence of a bronchial artery closely following the pulmonary artery .
The diameter of this bronchial artery was much too large for it to be a mere vasa vasorum ( figs. 16 , 23 , 24 ) .
In distal regions its diameter would be one-fourth to one-fifth that of the pulmonary artery .
This vessel could be followed to the parenchyma where it directly provided bronchial arterial blood to the alveolar capillary bed ( figs. 17 , 18 ) .
Also three other direct pathways of alveolar bronchial arterial supply were noted : via the pleura ; ;
through the interlobular septa ; ;
and along the terminal bronchiole ( figs. 14 , 17 , 18 , 19 ) .
One bronchial arteriolar-pulmonary arteriolar anastomosis was noted at the terminal bronchiolar level ( fig. 26 ) .
It is evident that many marked and striking differences exist between lungs when an inter-species comparison is made .
The significance of these differences has not been studied nor has the existence of corresponding physiologic differences been determined .
However , the dynamics of airflow , from morphologic considerations alone , may conceivably be different in the monkey than in the horse .
The volume and , perhaps , even the characteristics of bronchial arterial blood flow might be different in the dog than in the horse .
Also , interlobular air drifts may be all but nonexistent in the cow ; ;
probably occur in the horse much as in the human being ; ;
and , in contrast , are present to a relatively immense degree on a segmental basis in the dog where lobules are absent ( Van Allen and Lindskog , '31 ) .
A reason for such wide variation in the pulmonary morphology is entirely lacking at present .
Within certain wide limits anatomy dictates function and , if one is permitted to speculate , potential pathology should be included in this statement as well .
For example , the marked susceptibility of the monkey to respiratory infection might be related to its delicate , long alveolar ducts and short , large bronchioles situated within a parenchyma entirely lacking in protective supportive tissue barriers such as those found in types 1 , and 3 .
One might also wonder if monkeys are capable of developing bronchiolitis as we know it in man or the horse .
In addition , it would be difficult to imagine chronic generalized emphysema occurring in a cow , considering its marked lobular development but , conversely , not difficult to imagine this occurring in the horse or the dog .
Anatomically , the horse lung appears to be remarkably like that of man , insofar as this can be ascertained from comparison of our findings in the horse with those of others ( Birnbaum , '54 ) in the human being .
The only area in which one might find major disagreement in this matter is in regard to the alveolar distribution of the bronchial arteries .
As early as 1858 , Le Fort claimed an alveolar distribution of the bronchial arteries in human beings .
In 1951 , this was reaffirmed by Cudkowicz .
The opposition to this point of view has its staunchest support in the work of Miller ( '50 ) .
Apparently , however , Miller has relied heavily on the anatomy in dogs and cats , and he has been criticized for using pathologic human material in his normal study ( Loosli , '38 ) .
Although Miller noted in 1907 that a difference in the pleural blood supply existed between animals , nowhere in his published works is it found that he did a comparative study of the intrapulmonary features of various mammalian lungs other than in the dog and cat ( Miller , '13 ; ;
'25 ) .
The meaning of this variation in distribution of the bronchial artery as found in the horse is not clear .
However , this artery is known to be a nutrient vessel with a distribution primarily to the proximal airways and supportive tissues of the lung .
The alveoli and respiratory bronchioles are primarily diffusing tissues .
Theoretically , they are capable of extracting their required oxygen either from the surrounding air ( Ghoreyeb and Karsner , '13 ) or from pulmonary arterial blood ( Comroe , '58 ) .
Therefore , an explanation of this alveolar bronchial artery supply might be the nutritive requirement of an increased amount of supportive tissue , not primarily diffusing in nature , in the region of the alveolus .
If this be true , the possibility exists that an occlusive lesion of the bronchial arteries might cause widespread degeneration of supportive tissue similar to that seen in generalized emphysema .
One would not expect such an event to occur in animals possessing lungs of types 1 , or 2 .
The presence of normally occurring bronchial artery-pulmonary artery anastomoses was first noted in 1721 by Ruysch , and thereafter by many others .
Nakamura ( '58 ) , Verloop ( '48 ) , Marchand , Gilroy and Watson ( '50 ) , Von Hayek ( '53 ) , and Tobin ( '52 ) have all claimed their normal but relatively nonfunctional existence in the human being .
Miller ( '50 ) is the principal antagonist of this viewpoint .
In criticism of the latter's views , his conclusions were based upon dog lung injection studies in which all of the vascular channels were first filled with a solution under pressure and then were injected with various sized colored particles designed to stop at the arteriolar level .
As early as 1913 Ghoreyeb and Karsner demonstrated with perfusion studies in dogs that bronchial artery flow would remain constant at a certain low level when pressure was maintained in the pulmonary artery and vein , but that increases in bronchial artery flow would occur in response to a relative drop in pulmonary artery pressure .
Berry , Brailsford and Daly in 1931 and Nakamura in 1958 reaffirmed this .
Our own studies in which bronchial artery-pulmonary artery anastomoses were demonstrated , were accomplished by injecting the bronchial artery first with no pressure on the pulmonary artery or vein , and then by injecting the pulmonary artery and vein afterwards .
It is distinctly possible , therefore , that simultaneous pressures in all three vessels would have rendered the shunts inoperable and hence , uninjectable .
This viewpoint is further supported by Verloop's ( '48 ) demonstration of thickened bronchial artery and arteriolar muscular coats which are capable of acting as valves .
In other words , the anastomoses between the bronchial artery and pulmonary artery should be considered as functional or demand shunts .
In addition , little work has been done on a comparative basis in regard to the normal existence of bronchial artery-pulmonary artery anastomoses .
Verloop ( '48 ; ;
'49 ) found these shunts in the human being but was unable to find them in rats .
Ellis , Grindlay and Edwards ( '52 ) also were unable to find them in rats .
Nakamura ( '58 ) was unable to demonstrate their existence , either by anatomic or physiologic methods , in dogs .
The possibility that the absence or presence of these shunts is species-dependent is therefore inferred .
Certainly , the mere fact of failing to demonstrate them in one or another species does not conclusively deny their existence in that species .
It is , however , highly suggestive and agrees well with our own findings in which we also failed to demonstrate normally occurring bronchial artery-pulmonary artery shunts in certain species , especially the dog .
In conclusion , these findings suggest the need for a comparative physiology , pathology , and histology of mammalian lungs .
In addition , a detailed interspecies survey of the incidence of generalized pulmonary emphysema in mammals would be interesting and pertinent .
Also , for the present , great caution should be exercised in the choice of an experimental animal for pulmonary studies if they are to be applied to man .
This is especially so if the dog , cat or monkey are to be used , in view of their marked anatomical differences from man .
Finally , it is suggested that in many respects the horse lung may be anatomically more comparable to that of the human than any other presently known species .
The main subgross anatomical features of the lungs of various mammals are presented .
A tabulation of these features permits the lungs to be grouped into three distinctive subgross types .
Type 1 , is represented by the cow , sheep , and pig ; ;
type 2 , , by the dog , cat , and monkey ; ;
type 3 , , by the horse .
Lobularity is extremely well developed in type 1 ; ;
absent in type 2 ; ;
imperfectly developed in type 3 .
The pleura and interlobular septa are thick in types 1 and 3 .
The pleura is extremely thin in type 2 and septa are absent .
Arterial supply to the pleura in types 1 and 3 is provided by the bronchial artery , and in type 2 , by the pulmonary artery .
In types 1 , 2 and 3 the bronchial artery terminates in a capillary bed shared in common with the pulmonary artery at the level of the distal bronchiole .
In type 3 the bronchial artery also provides blood directly to the alveolar capillary bed .
True terminal bronchioles comprise the most frequent form taken by the distal airways in types 1 and 3 , although small numbers of poorly developed respiratory bronchioles are present .
Well developed respiratory bronchioles , on the other hand , appear to be the only form taken by the distal airways in type 2 .
In type 1 the pulmonary vein closely follows the course of the bronchus and the pulmonary artery from the periphery to the hilum .
This may be due to the heavy interlobular connective tissue barriers present .
In type 3 , this general relationship is maintained peripherally but not centrally where the pulmonary vein follows a more independent path to the hilum as is the case throughout the lung in type 2 .