OH tVI^RTSBRATE ANIMALS 



tortoises and birds a horny covering, and in the whales a row of horny 

 plates (in the upper jaw, the so-called baleen), takes the place 

 of the teeth which are wanting. Teeth are, as it were, ossified ' 

 papillas on the mucous membrane of the jaws ; they may also occur 

 on the palate, on the tongue, and, in fishes, on the branchial arches. 



The intestinal canal is of various width in different parts, and 

 receives various fluids, which are secreted by different glands. 

 Such a fluid is the saliva, which is mixed with the food during 

 mastication and deglutition, such also the bile and the pancreatic 

 juice. In all vertebrate animals the liver, besides arterial blood, 

 receives also much venous blood, which, as it returns from the 

 viscera, is taken up by a venous trunk (the portal-vein) , that like an 

 artery divides into branches to be distributed through the liver. 



From the food which has been changed by the action of the 

 stomach and of these secreted fluids, and is called chyme, the 

 chyliferous or lacteal vessels take up the nutritious part, and convey 

 it in a fluid state (called chyle) to the veins. Other vessels, in 

 addition, which resemble the lacteals, but which are not, like these, 

 spread over the intestinal canal, absorb from the different parts of 

 nearly the entire body a watery fluid (lymph), which becomes 

 mixed with the chyle. These vessels, with the preceding, make up 

 the lymphatic system, which appears proper to vertebrate animals 

 alone. They begin with blind extremities, and are formed of an 

 epithelium, which is covered by a fibrous membrane formed of fila- 

 ments running longitudinally and mutually crossing in form of a 

 net. On the outside of this membrane lies a covering of circular 

 fibres which pass into those of the surrounding connective tissue. 

 In the interior of the lymph-vessels, at least in most, are placed in 

 mammals and birds membranous valves, which favour the motion 

 of the fluid from the circumference of the body inwards, towards 

 the larger vessels. In mammals and in man almost the whole 

 fluid of this entire system meets in a principal stem, the thoracic 

 duct, which opens into the left subclavian vein. There are, how- 

 ever, smaller trunks also, which are situated on the right side, and 

 sometimes form connexions with other veins. The lymphatics 

 form repeated networks, and also as in warm-blooded animals, by 

 their rolling up and intertwining, glands improperly so named 

 (glandulce conglobatce). The spleen is also a part which is found 

 in vertebrate animals alone, and of which the presence and the 



