ORGANS OF CIRCULATION. 



59 



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The liver, distinguished in the higher grades of development by 

 its great size, is an appendage of the first part of the small intestine 

 (duodenum). The first trace of it is met with in the lower animals 

 in the form of a characteristically coloured part of the cellular 

 covering of the gastric cavity or intestinal wall (Coelenterata, 

 worms). In the higher animals it has at first the form of a small 

 blind sac (small Crustacea) ; this, by a process of branching, is con- 

 verted into a complicated struc- 

 ture composed of ducts and folli- 

 cles, which may become connected 

 together in very different ways 

 -so as to give rise to an apparently 

 compact organ. Nevertheless, it 

 must be remembered that, in the 

 different groups of animals, 

 glands, which differ both mor- 

 phologically and physiologically, 

 are included under this term, 

 "liver." While in the Verte- 

 brata the liver, as a bile-pro- 

 ducing organ, possesses no known 

 relation to digestion, in the In- 

 vertebrata the secretions of many 

 glands, which are generally called 

 " liver," but which would be 

 more appropriately termed hepato- 

 pancreas, exercise a digestive 

 action upon starch and albumen, 

 and at the same time contain 

 bye-products and colouring mat- 

 ters similar to those found in the 

 bile of Vertebrates (Crustacea, 

 Mollusca). 



The Organs of Circulation. The nutrient material or chyle re- 

 sulting from digestion is distributed by a system of spaces to all 

 parts of the body. Excluding the Protozoa, in which the distribution 

 of nutrient material is effected in the same manner as in the cell or 

 tissue unit, the simplest form of vascular system in animals with 

 cellular tissues, i.e., in the Metazoa, is found in the Coelenterata. 

 In these animals the digestive cavity itself extends to the extreme 

 periphery of the body, and serves to distribute the nutritive fluids 



Cue f, 



FIG. 50.- Alimentary canal of Mnn. Oe, 

 oesophagus ; JI/, stomach ; L, spleen ; H, 

 liver; Gb, gall bladder; P, pancreas; 

 Hit, duodenum receiving the bile and pan- 

 creatic ducts ; Jl, ileum ; Co, colon ; Coe, 

 esecum with vermiform process, Pa; R, 

 rectum. 



