698 BLOOD-VASCULAR SYSTEM OF THE HORSE 
forward a variable distance (3-4 em.) on the deep face of the left scalenus muscle, 
bends inward and backward under the bicarotid trunk, and opens into the dorsal 
part of the origin of the anterior vena cava just behind the angle of junction of the 
jugular veins. The terminal bend is ampullate, and sometimes divides into two 
very short branches which open close together. 
Since the duct develops from a plexus of ducts in the embryo, considerable variatior. from 
the more usual course occurs. There is often a left duct which arises at the cisterna or at a var- 
iable point from the right duct, runs across the left imtercostal arteries parallel to the latter, and 
unites with it over the base of the heart or further forward. ‘The two are connected by cross- 
branches. In some cases the left duct is the larger, and there may indeed be none on the right 
side. Other variations are common. 
The chief tributaries of the thoracic duct are as follows: 
1. The two lumbar trunks (Trunci lumbales) are formed by the confluence 
of the efferent ducts of the lumbar glands, and commonly unite with each other 
and with the posterior intestinal trunk before opening into the cistern. 
2. The intestinal trunks (Trunci intestinales), two or three in number, receive 
the efferents of the lymph glands of the intestine, stomach, liver, and spleen.! 
In its course through the thorax the thoracic duct receives efferents from the 
intercostal, mediastinal, and bronchial glands. At the thoracic inlet it is joined by 
ducts from the prepectoral and right axillary glands, and by the left tracheal duct. 
The duct is provided with several pairs of valves; the best developed are at its 
termination. 
THE RIGHT LYMPHATIC DUCT 
This vessel (Ductus lymphaticus dexter)—when present—collects the lymph 
from the right side of the head, neck, and thorax, and from the right thoracic limb. 
It is commonly absent, being represented by a number of short ducts which termin- 
ate in the thoracie duct, the right Jugulo-brachial junction, or the origin of the 
anterior vena cava. When present in its typical form, it results from the con- 
fluence of efferent ducts from the right axillary and posterior cervical lymph glands 
with the right tracheal duct. It lies on the deep face of the scalenus muscle, above 
the terminal part of the right jugular vein. It is more or less ampullate and usually 
opens into the anterior vena cava to the right of the thoracic duct. It may be 
connected with the latter by anastomoses or may join it. 
The duct is very variable in form and in regard to its afferents. Often it is a very short, 
irregular, and bulbous trunk; in some cases it is about an inch and a half (ea. 3-4 em.) in length 
and receives the tracheal duct at its terminal bend. The lymphatico-venous connections here 
have not yet been satisfactorily worked out by modern methods. 
THE LYMPH GLANDS AND VESSELS OF THE HEAD AND NECK 
1. The mandibular or submaxillary lymph glands (Leg. mandibulares s. sub- 
maxillares) (Figs. 265, 587) are arranged in two elongated groups in the mandibular 
space along each side of the omo-hyoid muscles. The two groups are in apposition 
in front of the insertion of these muscles, and diverge posteriorly in the form of aV, 
extending backward about four or five inches (ca. 10-12 em.). They are covered 
by the skin and a thin layer of fascia and cutaneus muscle, and are therefore pal- 
pable. Anteriorly they are firmly attached to the mylo-hyoidei, but otherwise they 
are freely movable in the normal state. Each group is related laterally to the 
external maxillary artery and the anterior belly of the digastricus, ventrally to the 
external maxillary vein, and dorsally to the lingual and sublingual veins. 
They receive afferent vessels from the lips, nostrils, nasal region, cheeks, the 
1 The lymph vessels of these organs appear to vary considerably in their mode of termina- 
tion, and the arrangement needs further study. Some lymph vessels from the stomach, liver, and 
spleen open directly into the cistern. 
