Fine Structure and Innervation, Especially, Sensory Innervation of the Lungs of Guinea-pig and Flying-squirrel

Abstract

The major bronchial branches of guinea-pig are smaller in diameter than those of cat, rabbit or hedgehog, containing no bronchial gland or lymphonodule inside the one-rowed cartilages, but the circular bronchial muscularis in their wall is rather well developed, and the epithelium is a rather thick non-ciliated 3-4-rowed cylindrical one. The medium-sized bronchial branches show no essential difference in sturucture from the above, but are more simplified in construction, the cartilages being particularly worse developed, but on the mucosa, regularly arranged low longitudinal mucous folds are found in many places.In the minor bronchial branches, the muscularis is very thin and the cartilages are only rudimentary, but the mucous folds are still present, covered by a 1- or 2-rowed cylindrical epithelium. The bronchioli respiratorii are composed of a one-rowed low cylindrical epithelium and a thin propria and a small number of smooth muscle fibres are found around them. The alveolar ducts and sacs are of nearly the same structure as in the other animals. It is of interest to find the media of the v. pulmonalis formed of heart muscle tissue, as that of bat. The pleura visceralis also contains a thin heart muscle layer.The lung of flying-squirrel is far smaller than that of guinea-pig, and accordingly, the bronchial walls are more simply made. The alveoli in the alveolar walls, however, are somewhat larger than in guinea-pig. The major bronchial branches are lined by a 1-2-rowed non-ciliated cylindrical epithelium, and no bronchial glands but not a few lymphonodules are found inside the one-rowed thin cartilaginous pieces. The bronchial muscles here, however, are very poorly developed, being barely found between the cartilages only.In the medium-sized bronchial branches, the cartilages exist only sporadically, a well-developed circular musculature taking their place. The mucosa is covered by a 1-2-rowed cylindrical epithelium and has a thin propria, containing low mucous folds.The minor bronchial branches are somewhat simpler in construction than the medium-sized, only a few smooth muscle fibres being fuund around the branches formed of a one-rowed cylindrical epithelium and a very narrow propria. The bronchioli respiratorii with cylindrical or cubic epithelium contain no smooth-muscle fibres. In flying-squrrel too, the media of the v. pulmonalis is of heart-muscle tissue, but the visceral pleura contains neither smooth-muscle nor heart-muscle tissue.The fundametal or bronchial plexus in guinea-pig is better developed than in flying-squrrel, as may be expected from the above outline of the histological structure. The ganglia accompanying the plexus are especially well developed in the former. The ganglion cells are of sympathetic nature, but in some cases, their multipolarity is very far from apparent, especially in flying-squirrel. In guinea-pig, submucosal plexus derived from the fundamental plexus is also formed in the major bronchial branches, but in flying-squirrel, such a formation is at best very indefinitely observable. The bronchial plexus is composed of many fine vegetative fibres and a far smaller number of thick medullated sensory fibres. Most of the former end in terminal reticula in the bronchial muscularis, but a small part come into relation with the perivascular plexus and partake in the formation of the terminal reticula in the vascular walls.Some of the sensory fibres are comparatively fine, but some are thick enough, and even enormously thick ones are not rare in the bronchial branches of flyingsquirrel, though very scarce in those of guinea-pig.Peculiar branched sensory terminations probably concerned with the blood-pressure falling reflex found in the respiratory musculature are present also in the major and medium-sized bronchial branches with rather well-developed muscularis of guinea-pig. These are here, however, of very simple type.