AN ANOMALY OF THE THORACIC DUCT. 53 



The actual conditions which are present in the normal human embryo are shown in 

 figure 1 , which represents a cross-section through the region of the seventh thoracic vertebra 

 in an injected human embryo (No. 4G0, 21 mm., in the collection of the Carnegie Institu- 

 tion of Washington). The section shows the two azygos veins connected by a cross-anas- 

 tomosis passing dorsal to the aorta. The relatively large size of the embryonic veins is 

 obvious. In the angle between each azygos vein and the aorta there is a lymphatic vessel, 

 the one on the right side being the early thoracic duct. It is easy to imagine the obstruc- 

 tion to the growth of lymphatics, which would be present, were the connection between the 

 two veins ventral instead of dorsal to the aorta, especially in case the left vein and the 

 connecting branch were much larger than in the section shown. Such a condition must 

 have obtained in early embryonic stages in the case under discussion. 



This case furnishes a most striking instance of the dependence of anatomical structure 

 on mechanical factors. Out of the lymphatics of the subcutaneous tissue of the left body- 

 wall, which ordinarily develop into narrow vessels draining in two directions, into the axilla 

 and the inguinal region, with capillary or (at most) very narrow anastomosis, there has been 

 produced a large vessel through which passed all the lymph of the posterior half of the body, 

 while the thoracic duct, deprived of this drainage, is a minute vessel, hardly more than one- 

 tenth its normal size, extending posteriorly only as far as the ninth thoracic vertebra. More- 

 over, along the course of this subcutaneous duct, anterior to the ligamentum inguinale, there 

 is a definite widening, corresponding somewhat to the receptaculum chyli, which is missing. 

 That blood-vessels are governed in their growth by mechanical factors has been shown by 

 Thoma (6, 1893) who finds that an increase or decrease in the amount of blood-flow leads 

 to increase or decrease in the size of the vessel, while an increase or decrease in pressure 

 causes increase or decrease in the thickness of the wall. It is apparent that lymphatic 

 vessels respond similarly to mechanical forces, since the passage of a large amount of 

 lymph through an unusual route has led to the formation of a duct many times larger than 

 the vessels normally present, while the diminution in lymph flow through the thoracic 

 duct has been accompanied by a corresponding diminution in its size. 



REFERENCES. 



1. F. R. Sabin. 1913. The origin and development of the 4. W. Polinski. 1910. Untersuchungen iiber die Ent- 



lymphatic system. The Johns Hopkins Hospital Re- j wieklung der subcutanen Lymphgefasse der Siiuger, in 



ports. Monographs, New Series, No. 5, 1913. Sonderheit des Rindes. Extrait du Bulletin de 1' \< a 



2. F. R. Sabin. 1904. On the development of the super- ' - - de ™' e deS Sciences d « Cracovie, 1910 ' £ 313 : , , 



ficial lymphatics in the skin of the pig. American Jour- 5 " L T Mierzejewski. 1909 Beitrag zur Entwieklung , lea 

 nal of Anatomy, vol. I.., 1904, p. 193. Lymphgefasse-systems der Vogel. Extnut du Bulletin 



de 1 Acadenue des Sciences de ( racovie, 1909, p. 4/ 'J. 



3. F. R. Sabin. 1912. Development of the lymphatic ! 6. R. Thoma. 1893. Untersuchungen iiber die Histoge- 



system. In Manual of Human Embryology, by Keibel nese und HistomechanikdesGefasssvstems. Stuttgart, 



and Mall, vol. n. 1912. 1893. 



