TEE TEOBACIC DUCT. 635 



initial dilatation, or that of the affluent vessels composing it. This is the 



thoracic duct. We see it enter between the two pillars of the diaphragm, 



along with the aorta, deviating more or less to the right side of that vessel, 



and follow it thus to about the sixth dorsal vertebra, in passing to the outside 



of the right intercostal arteries, which it crosses, and beneath the great vena 



azygos, beside which it lies. Sometimes, however, we find it carried in this . 



first part of its course directly above the thoracic aorta, between the double 



series of intercostal arteries, and to the left of the vena azygos, which is 



then found immediately in contact with the right side of the aorta ; or it 



may even creep 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 flexure of the vena azygos to the 



left, to extend itself forward on the left side of the trachea, but often also 



on the right side. It afterwards places itself between the two axillary 



arteries, crosses the interval comprised between the prepectoral glands, 



emerges from the chest, and terminates in a manner to be indicated 



hereafter. 



Termination. — The terminal extremity of the thoracic duct is always 

 provided with a dilatation 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, whose length we 

 cannot estimate 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 junc- 

 tion of the two jugulars. 'The thoracic duct rarely opens elsewhere ; though 

 the fact that it does so at times is exemplified in a specimen in the museum 

 of the Lyons School, in which the embouchure of the duct is placed 

 between the termination of the left jugular and that of the corresponding 



axillary vein. •■ ■■ • 



Varieties in SoUpeds.—'' The thoracic duct is far from always showing 

 itself in Solipeds as I have described it, but m its course and insertion 

 presents a great number of variations which we will now pass in review. 



" The single canal is separated sometimes, .at a part of its length, into two 

 branches, which, after proceeding parallel to each other, soon unite to form 

 a single vessel. This division usually takes place at the base of the heart, 

 at the place where the lymphatics of the bronchial and oesophageal glands 

 enter • it forms a ring whose diameter is often not more than four-tenths ot 

 an inch, or an eUipse whose larger axis is from four to eight-tenths of an m ch 

 We see this produced once, twice, and even thrice on the anterior haltot 

 the canal, which becomes simple at its termination as it was at its origin. 

 The spaces circumscribed by the bifurcations constitute what have been 



termed the insulse. . ■, j -un j> 



" The canal, instead of remaining single, very often becomes double trom 

 its commencement (Fig. 300). 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 canals 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, transverse anastomosing branches. Eeachmg to ten, 

 eight and sometimes even to two inches from their openmg mto the jugular 

 gulf the two canals approach each other, and become confounded into a single 



