RESPIRATION. 



273 



system : " the secoiv^ pulmonary, or small 

 circulation." The -nutrition of some parts of 

 these organs is sustained by another order 

 of vessels, distinct from these. 



The pulmonary artery, conducting venous 

 blood, and proceeding from the right ventricle, 

 fe the channel by which the blood destined to 

 be arterialised is conveyed to the lungs. It 

 is circumscribed in its distribution to the area 

 of the true pulmonary tissue as distinguished 

 from the bronchial. The plexus formed by 

 its branches is emphatically the rete mirabile. 

 The branches of the pulmonary artery 

 follow those of the bronchi as far as the 

 origin of the'intercellular passages ; a point at 

 which they assume an irregular course over 

 and between the cells. A lobule of the lung 

 receives, with great regularity, only a single 

 ramusculus from the pulmonary artery. It is 

 not so large in size as the bronchial tube 

 which it accompanies. Within the lobule, the 

 artery coincides with the tube in its divisions, 

 which are here more intimately bound to- 

 gether than at the extralobular stages of their 

 course. It was supposed by Bourgery that the 

 artery formed a framework of vessels around 

 the tube. This is not the case. Of course, 

 many of the branches of the pulmonary artery 

 course between the lobules in order to reach 

 others more distantly situated. Reisseissen 

 conceived that he had traced a ramuscule ot 

 the pulmonary artery to the root of each 

 " vesicle," describing a venule on the other 

 side. Krause supposed that each individual 

 cell, with unvarying constancy, had its artery 

 and vein, and intermediate plexus.* Berres 

 believed that each cellule presented, on its 

 circumference, a great many facets, like the 

 eyes of insects, each facet having its own 

 plexus. Rossignol -j* embraces the views of 

 Krause which assign a separate arteriole and 

 venule to each cell. In the accompanying 

 figure, taken from the essay of Adriani, and 

 drawn from a preparation by Schroeder van 

 der Kolk himself, the branches of the pulmo- 

 nary artery are seen to run, not only between 

 the ultimate air-cells, but in many instances 

 through the very centre of the walls. 



With this view the exact description of 

 Mr. Rainey coincides : " In the mammal the 

 number of capillary plexuses is not, as some 

 have supposed, the same as that of the air- 

 cells ; that is to say, a terminal artery does 

 not divide into a plexus at any particular part 

 of a cell, its branches uniting for the com- 

 mencement of a vein on the opposite part. 

 On the contrary, one plexus passes between 

 and supplies several cells. In the interior of 

 the lung the exact extent of an individual 

 plexus cannot be determined, in consequence 

 of the removal of some part of it by the sec- 

 tion necessary for its exhibition. But, on 

 the surface of the lung, where the extent of 

 these plexuses, in relation to the cells over 

 which they ramify, can be easily made out, 

 an individual plexus may be seen to spread 



* Huschke, Encycl. Anat. Splanchnologie, 

 p. 233. 



\ Op. cit. p. 66. 

 Supp. 



over an area of ten or twelve cells in some 

 parts and in fewer on others, the exact num- 



Fig. 223. 



A. thin slice (near the pleura! surface) of the lung of 

 the Cow, with the pulmonary artery (a), and pul- 

 monary vein (c), injected. (After Schroeder Van 

 der Kolk.) 



a, large arterial trunk terminating abruptly in 

 small branches (6, b, ft), which travel between and 

 along the borders of the air-cells, and in the ulti- 

 mate capillaries e, e, by short trunks as shown at 

 d; /,/, foramina arising from the sections of in- 

 tercellular passages ; g, fibrous trabeculse, supporting 

 by their elasticity the cells, and preserving their 

 wall at a regulated tension, 



ber depending in some measure upon the size 

 of the cells." * 



Around the foramina and margins of the 

 cells very frequent anastomosis takes place be- 

 tween the minute branches of the pulmonary 

 artery. With reference to this artery, it should, 

 however, be stated, that it differs from all other 

 arteries in the extremely infrequent inoscula- 

 tions which occur between its secondary and 

 tertiary branches and that its blood mingles 

 with that of no other vessel ; it is poured en- 

 tire into the pulmonary veins. " The trunk " 

 of a vessel is most certainly very seldom seen 



* Op. cit. p. 7. 



