454 MANUAL OF HISTOLOGY. 



being sufficient to allow of the easy passage of the blood-cells. When 

 the pulmonary vesicle is contracted, or but very slightly distended, they 

 appear too long for the extent of surface to be covered by them, and pro- 

 ject in the form of loops and tendril-like convolutions, pushing before them 

 a portion of the delicate lining membrane of the alveolus (fig. 433, d). 



But when the air-cells are strongly distended, these capillaries assume 

 a much straighter direction, while the loops and projections into the 

 vesicles disappear in a corresponding degree. 



In muscle, also, which is constantly undergoing change in length, we 

 find the same provision of nature. When contracted the longitudinal 

 tubes of its capillary network assume a spiral course ; when relaxed, on 

 the other hand, they appear straight. 



As to the walls of the capillaries, there is nothing remarkable about 

 them. They are usually nucleated, and may easily be resolved into the 

 well-known vascular cells (fig. 356, p. 363). 



The meshes bounded by these tubes are very close, even in lungs 



which have been previously inflated 

 (figs. 433, 434, 435). They may be 

 more or less round or angular. They 

 have a diameter of from P 0393 to 

 0-0293 mm. That in the uninflated 

 organ the meshes will be found much 

 smaller than in the inflated, owing 

 to their shrinking together, is quite 

 evident. 



The capillaries, further, of adjacent 

 alveoli intercommunicate very exten- 

 sively. 



But the fine twigs of the pulmonary 

 artery also, curious to say, form in 



Fig. 435. Pulmonary vesicle from the calf, a, .f ., ,. .-, r J M 



large blood-vessels traversing the septa of the another situation a wide-meshed capil- 



alveoli; 6, capillary network; c, epithelial } ary ne twork, namely, Under the 



pleura. Here they communicate with 

 the terminal tubes of the bronchial arteries. 



The pulmonar y veins take their origin from ths capillary networks just 

 described, with scattered twigs in the interalveolar septa. The con- 

 fluence of these produqes larger trunks, which accompany the bronchi 

 and ramifications of the pulmonary artery back to the root of the organ. 



The bronchial arteries, giving off as a rule a single branch to each of 

 the air passages, supply numerous twigs in the root of the lung to the 

 larger vascular trunks, the lymphatic glands of that neighbourhood, and 

 the connective-tissue between the lobuli and under the pleura. In the 

 walls of the bronchi and their ramifications they are resolved into an 

 external loose network of capillaries for the muscular tissue of the part, 

 and- an internal and much denser for the mucous membrane. In the 

 latter, however, there is, besides, another coarser and more superficial 

 capillary network, which does not appear to communicate in any way 

 with that of the bronchial arteries. It belongs to the respiratory system 

 proper, and may be injected easily from the vena pulmonalis, with diffi- 

 culty from the arteria pulmonalis, and not at all from the bronchial arte- 

 ries. From this we may infer that its radicals spring from the respiratory 

 capillary network (Herile). 



The arrangement of the bronchial veins, further, is peculiar. These pro- 



