i862 



HUMAN ANATOMY. 



Fig. 1587. 



The latter, from their relation to the principal stem of the pulmonary artery which 

 accompanies the chief air-tube, are regarded as dorsal and ventral. They alternate 

 with one another, and usually number four in each series ; not infrequently, how- 

 ever, the third dorsal bronchus fails to develop, thereby leading to a corresponding 

 reduction and asymmetry in the series. In the left lung the first dorsal bronchus 

 springs from the corresponding ventral bronchus instead of the chief tube, as on 

 the right side. This arrangement is probably associated with the fusion of the 

 upper and middle lobes in the left lung. 



The secondary bronchi elongate and give origin to tertiary bronchi, and these, 

 in turn, to air-tubes of lesser calibre, until the ramifications end as terminal bronchi 

 and the associated divisions — atria, air-sacs, and alveoli — of the lung-unit. Since 

 the fore-gut is clothed with entoblast, it is evident that the lining of the respiratory 

 tract is derived from the same germ-layer. At first the outpouchings of the respira- 

 tory tube are surrounded by relatively thick masses of 

 mesoblastic tissue. Since the growth of the latter fails 

 to keep pace with the increasing mass and complexity 

 of the bronchial tree, the intervening mesoblast becomes 

 greatly reduced. Coincidently the mesoblast becomes 

 vascular and rich net-works of blood-vessels appear 

 between the terminal divisions of the epithelial tubes, 

 later forming the chief constituents of the alveolar walls. 

 The mesoblastic tissue remains between the lobules as 

 the interlobular septa, as well as contributing all con- 

 stituents of the walls of the air-tubes except the lining 

 epithelial and its glandular derivatives, which are ento- 

 blastic. By the close of the fourth month of foetal life 

 the low columnar cells lining the trachea and bronchi 

 acquire cilia. The peripheral layer of the mesoblast 

 invaded by the lungs eventually becomes the investing 

 serous membrane, or pulmonary pleura, all parts of 

 which are of mesoblastic origin. Before inflation occurs 

 at birth, the lung-tissue is comparatively solid and re- 

 sembles in many ways a racemose gland. With the 

 expansion following the establishment of respiration, the 

 epithelial cells lining the ultimate air-spaces undergo 

 stretching, a majority of the small polygonal elements 

 becoming converted into the flat plate-like cells seen 

 in the functionating lung. 



The Larynx. — The pharyngeal end of the pri- 

 mary respiratory tract is surrounded in front and later- 

 ally by a U-shaped ridge, known as the furcida, anterior 

 to which lies the paired posterior anlage of the tongue. 

 The anterior portion of this ridge forms a median ele- 

 vation from which is formed the epiglottis ; the lateral 

 portions constitute the arytenoid ridges which bound the laryngeal aperture at the 

 sides. During the fourth month a furrow on the median side of the arytenoid ridges 

 marks the first appearance of the ventricle of the larynx, the margins of the groove 

 later becoming the vocal cords. About the eighth week the cartilaginous framework 

 is indicated by mesoblastic condensations. The thyroid cartilage consists for a time of 

 two separate lateral mesoblastic plates, in each of which cartilage is formed from two 

 centres. These are regarded as representing the cartilages of the fourth and fifth 

 branchial arches. As development proceeds the cartilages formed at these centres 

 fuse and extend ventrally until they unite anteriorly in the mid-line. Chondrification 

 is completed comparatively late, and when incomplete or faulty may result in the 

 production of an aperture, — the thyroid foramen. The anlages of the cricoid and 

 arytenoid cartilages are at first continuous, but later become differentiated by the 

 appearance of a centre of chondrification for each arytenoid and an incomplete ring, 

 for a time open behind, for the cricoid. The latter thus resembles in development 

 a tracheal ring, with which it probably morphologically corresponds. The cartilages 



Reconstructions of developing 

 hronchial tree. A, fourtli week; B^ 

 beginning of fifth week ; C, close of 

 fifth week. {His-Merkel.) 



