512 



OF SECRETION. 



hand, among the Mollusca, the Liver attains a much greater development. 

 Instead of the follicles being prolonged into* tubes (which is the usual form of 

 the glandular system in Insects), they are very much increased in number, 

 and arranged on the sides of canals or efferent ducts, which either separately 

 pour their fluid into the intestine, or partially unite with each other before 

 doing so. The Liver thus acquires a tabulated character,^each lobe consisting 

 of a duct with its branching follicles ; and the whole organ forms a considera- 

 ble proportion of the mass of the viscera, and is evidently of great importance 

 in the economy of the animal. It is interesting to compare this complex struc- 

 ture with the very simple condition presented by the Liver in Insects ; and, 

 when we keep in view the relative amount of Respiration in the two groups 

 of animals, we are at once struck with the fact, that the development of the 

 Liver bears an inverse proportion to the opportunity afforded by the Respira- 

 tory organs for the aeration of the blood; it being peculiarly extended, when 

 these, either from their small size, or from their employment in an aquatic 

 medium, cannot perform their function with great activity. This conclusion 

 is confirmed in an interesting manner by the fact, that the Crustacea, which 

 have the general organization of Insects, but which inhabit the water and 

 breathe by gills instead of by a complex system of air-tubes, possess a Liver 

 corresponding in form and in degree of development with that of the Mollusca. 

 654. In the Vertebrated Sub-kingdom, we may trace the operation of the 

 same principle ; but the internal structure of the Liver, in the adult condition 

 at least, is less easily demonstrated than it is in the lower classes ; owing to its 

 increased complexity of structure, and the closer union between its different 

 parts. In Fishes and Reptiles, the Liver is of considerable size, and seems to 

 perform a very important part in the decarbonization of the blood : its form is 

 adapted to that of the cavity in which it is lodged, sometimes one lobe only being 

 developed. In Birds, on the other hand, whose respiration is so much more 



[Fig. 134. 



The inferior or concave surface of the Liver, showing its subdivisions into lobes ; 1, centre of the right 

 lobe; 2, centre of the left lobe ; 3, its anterior, inferior or thin margin ; 4, its posterior, thick or diaphragmatic 

 portion; 5, the right extremity ; 6, the left extremity ; 7, the notch in the anterior margin ; 8, the umbilical 

 or longitudinal fissure; 9, the round ligament or remains of the umbilical vein; 10, the portion of the sus- 

 pensory ligament in connection with the round ligament; 11, pons hepatis, or band of liver across the um- 

 bilical fissure ; 12, posterior end of longitudinal fissure ; 13, 14, attachment of the obliterated ductus venosus 

 to the ascending vena cava; 15, transverse fissure; 16. section of the hepatic duct; 17, hepatic artery; 18, its 

 branches; 19, vena portarum ; 20, its sinus, or division into right and left branches; 21, fibrous remains of 

 the duclus venosus; 22, gall bladder; 23, its neck; 24, lobulus quartus; 25, lobulus spigelii; 20. lobulus 

 caudatus ; 27, inferior vena cava; 28, curvature of liver to fit the ascending colon ; 29, depression to fit the 

 right kidney; 30, upper portion of its right concave surface over the renal capsule; 31, portion of liver 

 uncovered by the peritoneum ; 32, inferior edge of the coronary ligament in the liver ; 33, depression made 

 by the vertebral column.] 



