722 THE LYMPHATICS IN PARUCULAM. 



pass to the right of that vein, concealing the greater part of it from sight. 

 Leaving the above-mentioned dorsal vertebra, the thoracic duct abandons the 

 aorta and crosses the flexm-e of the vena azygos to the left, to extend itself 

 forward on the left, but often also on the right side of the trachea. It after- 

 wards places itself between the two axillary arteries, crosses the interval between 

 the pre-pectoral glands, emerges from the chest, and terminates in a manner to 

 be indicated hereafter. 



Termination. — The terminal extremity of the thoracic duct has always a dila- 

 tation analogous to that which exists at its origin, though much smaller, better 

 circumscribed, and less irregular — a dilatation which opens into the anterior 

 vena cava sometimes by a single orifice furnished with valves, at other times by 

 two very short branches, the length of which cannot be estimated at more than 

 the fifth part of an inch, and which are also valvular at their entrance. The 

 point where this entrance takes place is nearly always at the summit of the vena 

 cava, and precisely at the point of junction of the two jugulars. The thoracic 

 duct rarely opens elsewhere ; though the fact that it does so at times is exempli- 

 fied in a specimen in the museum of the Lyons School, in which the opening of 

 the duct is placed between the termination of the left jugular and that of the 

 corresponding axillary vein. 



Varieties in Solipeds. — " The thoracic duct is far from always showing itself 

 in Solipeds as I have described it, but in its course and insertion it presents a 

 great number of variations which we w^ill now pass in review. 



" The single canal is sometimes divided into two branches, which, after pro- 

 ceeding parallel to each other, soon unite to form a single vessel. This division 

 usually takes place at the base of the heart, where the lymphatics of the 

 bronchial and a-sophageal glands enter ; it forms a ring the diameter of which 

 is often not more than f of an inch, or an ellipse whose larger axis is from § 

 to f of an inch. We see this happen once, twice, and even thrice in the anterior 

 half of the canal, which becomes single at its termination as it was at its origin. 

 The spaces circumscribed by the bifurcations constitute what have been termed 

 the insuhe. 



" The duct, instead of remaining single, very often becomes double from its 

 commencement (Fig. 397). Then the two canals are sensibly equal, or one is 

 larger than the other. If they are unequal, it is usually the right which has the 

 advantage, though the contrary sometimes occurs. In any case, the two ducts 

 are isolated — one being to the right, the other to the left of the aorta. In 

 advancing towards the entrance to the thorax, they remain completely separated, 

 or communicate with each other by one or two — more or less voluminous, trans- 

 verse anastomosing branches. Reaching to ten, eight, and sometimes even to 

 two inches from their opening into the jugular gulf, the two ducts approach 

 each other, and become at last a single vessel. Their fusion generally takes 

 place at the base of the heart, and I have never seen them remain distinct 

 throughout their whole extent, to enter the vena cava separately. 



" Sometimes (Fig. 39k) there emanates from the gland, at the entrance to the 

 thorax, a long canal which proceeds parallel to the first, and joins it after a 

 retrogade course, near the crura of the diaphragm. 



" The thoracic duct, double for the greater part of its extent, from the time 

 it leaves the receptaculum chyli, occasionally ends by becoming triple. In this 

 case, the largest of the two canals is divided into tAvo branches ; then the three 

 canals, after pursuing a certain course, all join at the same point, or two are first 



