100 THE ANATOMY OF VEKTEBRATED ANIMALS. 



contractility. Except in the Marsii^ohrancMi, whicli have 

 round blood-corpuscles, the red corpuscles are oval. They 

 attain a larger size in the perennibranchiate Amphibia 

 than in any other Vertebrates. 



In Mammalia, the blood-corpuscles are also of two kinds, 

 colourless and red, the colourless possessing, and the red 

 being devoid of, nuclei. It is but very rarely that a nu- 

 cleated corpuscle, with a red coloxir especially developed 

 about the nucleus, is seen in Mammalian blood ; but such 

 cases do occur ; and, from this and other circumstances, it 

 is probable that the Mammalian red corpuscle is a free- 

 coloured nucleus. 



The colourless corpuscles of Mammalia are spheroidal, 

 and exhibit amseboid movements ; the red corpuscles are 

 Hatteued, visually circular, Ijut sometimes oval (Camelidce) 

 discs, devoid of contractility. 



The Lymphatic System. — This system of vessels consists, 

 chiefly, of one or two principal trunks, the thoracic duct, or 

 ducts, which underlie the vertebral column, and communi- 

 cate, anteriorly, with the superior venae cavse, or with the 

 veins which open into them. 



From these trunks, branches are given ofl", which ramify 

 through all parts of the body, except the bulb of the eye, 

 the cartilages, and the bones. In the higher Vertebrata, 

 the larger branches are like small veins, provided with 

 definite coats, and with valves opening towards the larger 

 trunks, while their terminal ramifications form a capillary 

 network ; but, in the lower Vertebrates, the lymphatic 

 channels assume the form of large and irregular sinuses, 

 which not vvnfrequently completely suiTound the great 

 vessels of the blood system. 



The lymphatics open into other parts of the venous 

 system besides the affluents of the superior cavae. In Fishes 

 there are, usually, two caudal lymphatic sinuses which open 

 into the commencement of the caudal vein. In the Frog, 

 four such sinuses communicate with the veins, two in the 

 coccygeal, and two in the scapular, region. The walls of 



