698 BLOOD-VASCULAR SYSTEM OF THE HORSE 



forward a variable distance (3-4 cm.) 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 iDehind the angle of junction of the 

 jugular veins. The terminal bend is ampuUate, 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 variation from 

 the more usual course occurs. There is often a left duct which arises at the cistema or at a var- 

 iable point from the right duct, runs across the left intercostal 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 bj^ 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 thoracic duct, the right jugulo-brachial junction, or the origin of the 

 anterior vena cava. When present in its tj^pical 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 maj' 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 (ca. 3-4 cm.) in length 

 and rc<'pi\'es the tracheal duct at its terminal bend. The lymphatico-venous connections here 

 have not yet been satisfactorily worked out by modem methods. 



THE LYMPH GLANDS AND VESSELS OF THE HEAD AND NECK 



1. The mandibular or submaxillary lymph glands (Lgg. 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 a V, 

 extending backward about four or five inches (ca. 10-12 cm.). They are covered 

 bjr the skin and a thin layer of fascia and cutaneus muscle, and are therefore pal- 

 pable. Anteriorly they are firmlj^ 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. 



