600 THE VEINS 



several inches in length. In form they may })e globular, ovoid and flattened, 

 elongated, or irregular. In certain situations they are aggregated into groups, and 

 a knowledge of the position of these and the territory drained into them is im- 

 portant. It is convenient, when possible, to indicate their position with regard to 

 arteries on the course of which they are placed. In color they are usually gray or 

 yellowish-brown in the dead subject, pink or reddish-brown during life, but this 

 varies according to their position and functional state. The bronchial glands 

 are often blackened l)y infiltration with carbon. The mesenteric glands are creamy 

 or white while the chyle is passing through, but pink at other times. \>ssels 

 which carry lymph to a gland are called afferent ; the efferent vessels which convey 

 it away are larger and fewer. Each gland has a depression, the hilus, at which the 

 blood-vessels enter and the efferent lymph vessels emerge. 



L)miph nodules or follicles (Noduli lymphatici) are minute masses of lymphoid 

 tissue which occur in certain mucous membranes. They may be solitary, as in the 

 solitary glands of the intestine, or aggregated into masses or patches, as in the tonsils 

 and the so-called Peyer's patches. 



The lymph nodule or follicle is the unit of structure of the lymph gland. It consists 

 essentially of an artery surrounded l)y a reticulum of connective tissue, the meshes of whicli 

 contain numerous lymphocytes. Siu'rounding this is a rich plexus of lymph vessels, forming the 

 so-called sinus, inclosed in some cases by a fibrous capsule. Tlic gland consists of a mass of 

 follicles, inclosed in a fibrous capsule, from which trabeculae pass in and unite tlie follicles. Be- 

 neath the capsule is the peripheral sinus, which consists of a very rich plexus of lymph vessels; 

 to this the afferent vessels pass at various points of the surface. In the cortical substance the 

 cells are in rounded masses, the cortical nodules, while in the medullary substance tliey lie 

 around the arteries, forming the so-called medullary cords. The medulla is redder than the cor- 

 tex, since it is more vascular; it contains the central lymph sinuses, which have a similar structure 

 to the peripheral sinus. 



The haemolymph glands differ from the lymph glands in color and structure. 

 They are of a deep red color, which is due to the high vascularity of the cortical 

 substance. The peripheral sinuses especially are greatly developed and contain 

 numerous red blood-cells. There is no clear division into cortical and medullary 

 substance, and the trabecule contain smooth muscle-cells. Some have afferent 

 and efferent lymph vessels and others do not. They resemlile the spleen in some 

 respects, but their significance is not yet clear. They are numerous in the ox and 

 sheep, much fewer in the dog, and apparently are absent in the horse. They occur 

 along the course of the aorta, in the perineal fat, at the portal fissure, and with 

 the gastric and mesenteric lymiih glands. In the ox they are also found under the 

 trapezius muscle, under the skin of the upper part of the flank, and in other places 

 less constantly. 



The tissue or lymph spaces are interstices of varying size between cells or 

 in the meshes of connective tissue. They contain a fluid derived from the blood- 

 plasma, which is usually calknl lymph. They are drained ])y the veins and lymph 

 vessels. The large serous sacs arc often included in this category. 



The exact relationship between the lymphatic capillaries and the tissue spaces is still a matter 

 of controvers3\ It is held by some that the lymph-vessels are in direct communication with the 

 tissue spaces, while otiiers maintain that the lympliatics are complete closed tubes. Comnumi- 

 cation between the spaces and ves.sels is in general very free. Mall has shown that granules in- 

 jected into the hepatic artery are returned l)y the !ymy)hatics as well as by the veins, and intra- 

 muscular injections will enter the lymph-vessels of the tendon in spite of the absence of lymphatics 

 in muscle. 



Lymphatic System of the Horse 



THE THORACIC DUCT (Figs. 428, 429) 



The thoracic duct(Ductus thoracicus)is the chief collecting trunk of the lymph- 

 atic syst(Mn. It Ix'gins as an elongated irregular dilatation, the cistema or recepta- 

 culum chyli, which is situated between the right side of the aorta and the right crus 



