550 



RESPIRATORY APPARATUS IN MAMMALIA. 



(The minute polygonal cells lining the air or pulmonary vesicles measure from 

 TffW to ^TD^ of an inch in diameter, and from ^winj <^o ^^5^ of an inch in thick- 

 ness. Between the vesicles is a tra- 

 p's- 328. becular tissue, mainly composed of 

 yellow elastic with a few muscular 

 fibres, some of which are united with 

 the lining membrane to strengthen it, 

 especially around the apertures of 

 communication between the adjoining 

 air-cells. 



The capillaiy plexuses are so ar- 

 ranged between the two layers forming 

 the walls of two adjacent cells, as to 

 expose one of their surfaces to each, 

 in order to secure the influence of the 

 air upon them. These networks are 

 so close, that the diameter of the 

 meshes is scarcely so great as that of 

 the capillaries which enclose them.) 



Vessels. — The lung is a very 

 vascular organ. The numerous rami- 

 fications it receives divide into two 

 orders — the functional and the nutri- 

 tive vessels. 



Functional vessels of the lung. — 

 The blood is returned from all parts 

 of the body by the veins, after losing, 

 along with its bright red colour, the properties which render it fit to maintain 

 the vitality of the tissues. It thus arrives at the right side of the heart, whence 

 it is propelled into the lung, there to be regenerated by mediate contact with the 

 air. It is the pulmonary artery which conveys this fluid into the parenchyma of 

 the organ, and by the pulmonary veins it is carried back 

 to the heart. The artery is at first divided into two branches, 

 which ramify, and finally terminate in dense capillary plexuses 

 upon the walls of the infundibula. The veins — innumerable 

 and attenuated at their origin, like the arterial capillaries 

 — terminate in from four to eight principal trunks, which 

 open into the left auricle of the heart. 



The branch of the pulmonary artery that enters each 

 lobule, is regarded as terminal — that is, it does not anas- 

 tomose directly with the arteries of the neighbouring lobules ; 

 on the contrary, the interlobular branches of the pulmonary 

 vein are in relation with the capillary network of the adjoin- 

 ing lobules. 



These two orders of vessels, which necessarily participate 

 in the physiological functions of the lung — like the vena 

 port* with the liver — are very justly distinguished from the 

 other arteries or veins, by the designation o( functional vessels. But though they 

 are so named, it must not be inferred that they are excluded from all partici- 

 pation in the acts of nutrition. 



CAST OF A PORTION OF THE BRONCHIAL DIVISIONS 

 AND SOME LOBULES AND ACINI, FROM THE LUNG 

 OF THE ASS. 



1, Bronchiole, IJ millimetre in diameter; 2, 2, 2, 

 ramifications of 1 millimetre ; 3, 3, 3, 3, sub- 

 lobular bronchioles ; 4, 4, 4, intra-lobular 

 bronchial divisions ; 5, 5, acini ; 6, 6, respiratory 

 canaliculi ; 7, 7, infundibula; 8. 8, 8, mould of 

 complete lobules. It was necessary to break 

 away a number of lobules in order to show these. 

 Magnified 5 diameters. 



Fig. 329. 



PLAN OF A PULMO- 

 NARY LOBULE. 



a, Bronchiole termi- 

 nating in a slight 

 dilatation, b ; c, air- 

 sacs, or infundibuli; 

 d, air or pulmonary 

 vesicles. 



