338. PHILOSOPHY OF ZOOLOGY. 
Mowro, succeeded in demonstrating their existence in birds,. 
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 is another class of vessels, termed 
Verns, 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 vene cava, 
and are either swperior 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 stems. 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 
