1 88 ELEMENTS OF MAMMALIAN ANATOMY 



■channels, as is found to be the case in lower vertebrates. The 

 theory that the lymphatic system is budded off directly from 

 the venous system is held by some investigators, however, the 

 theory outlined above seems best to agree with the results of 

 investigations in fishes (Allen, McClure), amphibians (Kamp- 

 meier) reptiles (Stromsten, Huntington) and birds (Miller). 

 The development of the lymphatic system is a very difficult 

 problem and much depends upon the kind of technique and its 

 interpretation. In the adult system as with the bloodvascular 

 system there are many variations, such as place and number 

 of lymphatico-venous taps, the persistence of lymph sacs (Fig. 

 84) or lymph plexuses etc. There are many problems relating 

 to the development, morphology and function of the lymphatic 

 system yet to be solved. 



DUCTLESS GLANDS 



The spleen, thyreoid gland, thymus gland, and suprarenal 

 (adrenal) bodies are ductless glands whose functions are imper- 

 fectly known. The largest of these is the spleen, which is 

 of a deep red color in a fresh specimen, and lies in the abdom- 

 inal cavity on the left side caudad to the stomach (Fig. 61). 

 It is about five centimeters long, two centimeters wide, and 

 less than a centimeter thick. It is composed of lymphoid 

 tissue of two forms, supported by connective-tissue trabeculae 

 which are merely prolongations of the enveloping capsule. 

 The two forms of tissue can be seen by cutting the organ trans- 

 versely. The dense lymphoid tissue appears as white spots 

 less than the size of a pin-head. They are the splenic nodules. 

 The intervening looser lymphoid tissue forms the greater part of 

 the spleen, and is known as the splenic pulp. The splenic artery, 

 a branch of the celiac axis, enters the spleen at its hilus and 

 divides into capillaries which terminate in irregular spaces, 

 thus permitting the blood to flow freely through the splenic 

 pulp, whence it is taken up by the capillaries of the splenic 

 vein leading to the portal vein. In embryonic life the spleen 



