944 HUMAN ANATOMY. 



with the receptaculum chyH. It must be remembered that embryologically what are usually 

 termed the origins of the thoracic duct are in reality its prolongations, that is to say, outgrowths 

 from it, so that possibilities for variation in these stems are abundant. 



In another respect the embryological history of the duct probably throws light upon its 

 anomalies. In the rabbit the spaces formed along the course of the left posterior cardinal \ein 

 frequently unite to form two more or less distinct, parallel stems, which together represent the 

 thoracic duct (Fig. 794). Whether this condition also exists in man is unknown, but if it does 

 then an explanation is afforded for one of the most frequent anomalies of the duct, namely, its 

 division in its lower part into two parallel stems which unite again after a longer or shorter inde- 

 pendent course. This condition is so frequent that it has been regarded as typical by some 

 authors ; usually the union of the two stems occurs at about the level of the seventh thoracic 

 vertebra, but occasionally they remain separate throughout the entire length of the thorax and 

 may be connected by transverse anastomoses. 



Another group of anomalies, probably having a quite different embryological basis, includes 

 cases in which there are either two distinct thoracic ducts, or else a single one which branches 

 in its upper part, one of the two stems in either case passing to the left subclavian vein and the 

 other to the right. This condition is due to the fact that the lymphatic system is symmetrical in 

 its embryological origin, a trunk arising in connection with the right azygos vein as well as with 

 the left. Ordinarily the left trunk, developing more rapidly than the right, becomes the thoracic 

 duct, while the right outgrov\th remains short and forms the right lymphatic duct. Conditions 

 might occur, however, in which the right trunk would undergo a more extensive development 

 and either unite with the left trunk or grow downward to form a second thoracic duct, thus 

 producing the conditions under discussion. A further modification along the same line would 

 lead to the development of the thoracic duct from the right trunk, the left giving rise only to 

 a short lymphatic duct, an exact reversal of the normal arrangement being thus produced. 

 Several such cases have been recorded, and it is interesting to note that they frequently 

 accompany abnormalities of the aortic arch, such as the origin of the right subclavian from the 

 descending portion ; the anomaly also occurs, however, independently of any variation in the 

 blood-vessels. 



Considerable variation exists in the level to which the arch of the thoracic duct rises in the 

 neck, and it is stated that it may lie anywhere between the levels of the fifth cervical and first 

 thoracic vertebrae. 



Likewise, variations in the mode of termination of the thoracic duct are often observed. 

 It may open into the subclavian vein at some distance from the junction of the internal jugular, 

 or, occasionally, into its posterior surface, and not infrequently it divides near its termination 

 into two or more stems (Fig. 795), which may open into the internal or the external jugular 

 or into the azygos or vertebral veins as well as into the subclavian. The connection with the 

 azygos vein is probably of frequent occurrence. 



Practical Considerations. — The thoracic duct may be obstructed by (a) 

 aneurism of the arch of the aorta ; (^) enlarged mediastinal nodes (tuberculous, 

 lymphadenomatous, or carcinomatous) ; (<:) mediastinal neoplasms — especially if in 

 the anterior mediastinum ; (</) exophthalmic goitre (very rarely) ; (<?) thrombosis 

 of the left innominate vein or of the subclavian at its junction with the internal jugular ; 

 (y) tricuspid incompetence (through backward pressure) ; { g^ cardiac hyper- 

 trophy ; (/^} dense pancreatic growths (Agnew) ; (?) thrombosis (tuberculous) of 

 the duct itself ; (y) filarial disease (obstruction by the parent worms) ; (^k) cicatri- 

 cial contraction or adhesion involving the duct ; (/) disease (tuberculous, carcino- 

 matous) of the walls of the duct. 



The duct may be injured (a) during operations — as for growths or enlarged 

 glands — or by stab or bullet wounds (usually in its cervical portion) ; or (d) by 

 grave trauma, as fracture dislocation of the spine (usually in the thoracic or abdomi- 

 nal portion), or violent compression of the thorax ; or (c) by muscular effort or 

 during a paroxysm of vomiting (Busey), or whooping-cough (Wilhelm). 



The fact that the duct as a rule extends upward but little if at all above the 

 level of the junction of the internal jugular and subclavian renders operative injury of 

 it rare, but as it occasionally is found higher, and may even extend to 5.5 cm. 

 (234^ in.) above the upper border of the sternum, its possible presence and its rela- 

 tions and variations (vide supra) should not be forgotten during extensive operations 

 at the base of the neck on the left side. 



The results of obstruction of the. thoracic duct are {a) increased pressure 

 and dilatation of the vessels behind the obstruction ; {b) the establishment of 

 collateral circulation and entrance of lymph into the general circulation; or — if 

 such collateral circulation is not established — {c) leakage by transudation into 

 the surrounding tissues, into the pleural cavity (rare), or into the peritoneal 

 cavity ; or (a?) rupture of the duct or its tributaries. The stomata of the thin-walled 



