COMPARATIVE ANATOMY OF PANCREAS. 97 



CHAPTER VI. 



COMPARATIVE ANATOMY AND PHYSIOLOGY OF THE 



PANCREAS. 



Among the invertebrates no lymphatic system has been discovered, and the 

 existence of a pancreatic gland has not as yet been satisfactorily shown. 



Siebold considers two thick walled ca?ca, lined with ciliated epithelium, and 

 opening into the beginning of the stomach, in many Rotatoria, a rudimentary 

 pancreas. Hunter, Grant, Owen, Siebold, and Rymer Jones, consider the pale- 

 yellow ramified tubes, which, in many species of Cephalopoda, are appended to the 

 hepatic ducts as true representatives of the pancreatic glands of the higher animals. 

 These, however, have not been definitely proved to be pancreatic glands, for no 

 comparative anatomist or physiologist has as yet described the special character 

 and offices of their secretion. 



In the four great classes of vertebrate animals the circulatory system is completely 

 separated from the digestive cavity, all the organs are highly developed, and the 

 existence of a special system of absorbents appears to be absolutely necessary for 

 the preservation of the integrity of the animal fluids, and also for the absorption 

 of fatty matters, which are of great importance in the maintenance of animal 

 temperature. 



In Fishes, the development and perfection of the pancreas corresponds in no 

 degree with the position occupied by different individuals in the classification of 

 naturalists. The most superficial examination of the gland under consideration 

 will show that many animals, of an exceedingly simple structure, often have 

 individual organs more highly developed than those which stand far above them in 

 physical and mental constitution. Thus in the lowest orders of the cartilaginous 

 fishes, the Cyclostomi and Plagiostomi, this gland resembles, in all respects, that of 

 the more highly organized mammals. In the Sturiones, its structure is somewhat 

 simplified, and, in the majority of Osseous Fishes, it is reduced to its rudimentary 

 form, consisting of cosca, varying in number in different species, and opening into 

 the duodenum below the circular valve of the stomach, whilst in others, as observed 

 by Cuvier in the Conger Eel, Pike, and Carp, and by Miiller in the Oplmurus 

 serpens, it is intimately associated with the internal mucous membrane, consisting of 

 simple follicular depressions lined with the peculiar cells constituted to secrete the 

 pancreatic fluid. It cannot, therefore, be asserted as a universal rule, without any 

 exception, that there is a regular progression in the development of the different 

 organs in animals, corresponding to the position which they occupy in the scale of 

 creation. This may be illustrated by comparing together the rudimentary pancreas 

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