338 PHILOSOPHY OF ZOOLOGY. 



MONRO, succeeded in demonstrating their existence in birds y , 

 reptiles, and fishes. Previous to these discoveries, the veins 

 were supposed to perform the office of absorbents. In birds 

 the chyle is transparent ; so that the lacteals can only be 

 distinguished from the lymphatics by their origin and of- 

 fice. There are no mesenteric glands. The lymphatics, 

 however, possess numerous conglobate glands. The tho- 

 racic duct terminates in two branches. In reptiles, the 

 thoracic duct is double as in birds, but neither the lacteals 

 nor lymphatics appear to possess any glands. In the tur- 

 tle these systems frequently unite, by anastomosing 

 branches. In the crocodile, according to HEWSON, the 

 chyle is white. In fishes, the absorbing system is equally 

 simple as in reptiles ; but the vessels have no valves. In 

 the lower animals, as the mollusca and annulosa, the exist- 

 ence of lacteals or lymphatics has not been demonstrated. 

 The veins are considered as officiating in their stead. 



Besides the lacteals and lymphatics which we have now 

 been considering, there i& another class of vessels, termed 

 VEHCS, by means of which the pulmonic system completes 

 its operations. These veins are connected by their radicles, 

 with the extremities of the arteries, and probably also with 

 the lymphatics. These radicles, by their union, form 

 branches, which frequently anastomose. In the mammalia 

 these branches unite into two trunks, termed vena cav<c, 

 and are either superior or inferior, according as they col- 

 lect the blood from the atlantal or sacral extremities. 



The structure of the veins has been investigated chiefly 

 in the larger steins. The walls consist of three coats. The 

 external one is composed of a dense cellular substance. The 

 middle one is muscular, and consists of fibres, interwoven in 

 all directions. The internal covering is very thin and smooth 

 on its central surface. It assumes, in many places, the form 

 of loose folds> which act the part of valves, in preventing the 



