632 Comparative Morphology of Chordates 



close to the heart, is achieved by retention of the right precava along 

 with the lefl aortic trunk. The 4 loss of the left precava is an extreme 

 case of the simplification of the vascular system b\ concentration and 

 reduction of the main channels. 



Lymphatics 



The mammalian lymphatic system is characterized by absence of 

 ''lymph-hearts," presence of numerous valves in the larger trunks, and 

 the development of many large and definitely localized lymph-glands 

 (Fig. 66). 



In all vertebrates the walls of the larger lymph-vessels contain a 

 thin layer of muscle. In anamniotes and reptiles certain localized 

 regions of the larger lymphatics possess a thicker muscular layer 

 rendering the region more strongly contractile — a "lymph-heart." In 

 or near these "hearts" are valves similar to those found in veins. 

 Contraction of the lymph-hearts drives lymph toward the veins into 

 which the lymphatics open, the valves being so set as to prevent flow 

 in the reverse direction. Such lymph-hearts are especially well devel- 

 oped and numerous in urodele amphibians, fewer in frogs and toads, 

 in reptiles reduced to a single pair in the dorsal pelvic region, and in 

 the bird embryo to a single pelvic pair which do not persist in the 

 adult. In mammals lymph-hearts ordinarily do not occur, although on 

 some of the lymphatics of bats have been found some enlarged thick- 

 walled regions resembling lymph-hearts. 



The flow of the fluid in mammalian lymphatics is due to several 

 factors aside from contraction of the vessels themselves. Throughout 

 the body the volume of lymph is continually being increased by the 

 passage of blood-plasma through the very thin walls of the capillaries 

 into the minute intercellular spaces. Some initial pressure is therefore 

 maintained at all points of origin of the lymph. Blood-pressure is at a 

 minimum in the larger veins, and it is into these veins that the main 

 lymph-channels eventually open. Accordingly, the movement of lymph 

 is from a region of higher to one of lower pressure. An additional and 

 important factor in mammals is indicated by the usual absence of 

 lymph-hearts and the presence of valves set at short intervals along 

 the larger trunks (Fig. 480). When a bulky muscle such as those of 

 the body-wall and appendages contracts, pressure must be exerted on 

 vessels which are adjacent to it. Internal pressure in the lymphatics is 

 low. When external pressure is increased, a thin-walled lymphatic is 

 flattened and the lymph escapes from the compressed region in the 

 only direction permitted by the valves — that is, toward the veins. 

 Upon release from external pressure, the compressed region of the 

 lymphatic refills from the reverse direction. 



