246 LECTURE X. 



more ramified cfeca, with a concomitant increase of the vascular 

 supply, and a common covering or capsule, finally converts the ac- 

 cessory intestinal growths into a true parenchymatous conglomerate 

 gland ; as we see in the Holocephali and Plagiostomes, (preps. 776, 

 777.) : the i^apilliform termination of the duct of such a pancreas 

 is shown in the Selache ^ifig. 64. i. 



The existence of this highly developed form of pancreas over and 

 above the spiral intestinal valve may relate to the high organisation 

 of these Cartilaginous Fishes, and to the great development of the 

 organs of locomotion, occasioning the necessity for rapid and com- 

 plete digestion. But if we compare the few existing species of 

 heavily-laden Ganoid fishes, we shall again find good evidence of the 

 compensation for a pancreas by the extension of the intestinal mu- 

 cous membrane within the canal, the circumstances calling for a more 

 complete development of the digestive system in the predatory 

 Sharks and large-finned Rays not being present. Thus the Poly- 

 pterus, which has a spiral intestinal valve, has only one short pyloric 

 caecum {Jjg. 62. h) ; whilst the Lepidosteus, which has no spiral valve, 

 has a compact group of above a hundred small casca, which unite and 

 reunite to communicate by a few apertures with the commencement 

 of the duodenum. 



LECTURE X. 



VASCULAR SYSTEM OF TISHES. 

 THE ABSORBENTS. 



The assimilation of food in an animal body has a close analogy to a 

 chemical operation, the general result being the obtaining from a 

 combination of two substances a third distinct from both. If the 

 chemist operates on a solid substance, he first triturates and reduces 

 it to powder, then digests it in a solvent menstruum, next adds the 

 reagent, and if the result of the admixture be a precipitate, the su- 

 pernatant fluid is drawn or filtered off. So it is with the food : it 

 first undergoes the process of mastication in the mouth, next that of 

 solution in the stomach, then the chyme is mixed with the intestinal, 

 biliary, and pancreatic secretions, and lastly the chyle is filtered off" 

 from the faacal precipitate. The instruments of this separation and 

 conveyance of the chyle to the circulating organs are the ' lacteals :' 

 analogous vessels which take up the effete parts of the body are 

 called the ' lymphatics ;' both together constitute the 'absorbent 



