492 



DIGESTION IN THE INTESTINES. 



[CH. XXXII. 



a little higher in the series its formula is C 5 H n .COOH ; amido- 

 caproic acid is C 5 H 10 .NH2.COOH, which is also called leucine. 



According to the way in which the aniidogen is linked, a large 

 number of isomeric amido-caproic acids, all with the same em- 

 pirical formula, are theoretically possible. Some of these have 

 been actually prepared in the laboratory ; and chemical research 

 has shown that the amido-caproic acid called leucine formed 

 during digestion should be more accurately named a-amido- 

 isobutylacetic acid (CH 3 ) 2 CH.CH 2 .CH(NH,)COOH. 



Tyrosine is a little more complicated, as it is not only an 

 amido-acid, but also contains an aromatic radicle. Propionic 

 acid has the formula C 2 H 5 .COOH ; amido-propiouic acid is 



Fig. 402. Crystals of leucine and tyrosine. ^ x 216. 



C.jH 4 .NH,.COOH, and is called alanine. If another H in this is 

 replaced by oxy phenyl (C 6 H 4 .OH), we get C,H 3 .NH 2 .C 6 H 1 OH.COOH, 

 which is oxyphenyl-amido-propionic acid, or tyrosine. Leucine 

 and tyrosine are both crystalline ; the former crystallises in the 

 form of spheroidal clumps of crystals, the latter in collections of 

 fine silken needles (fig. 402). 



Secretory Nerves of the Pancreas. 



It has been known since the work of Claude Bernard in 1856 

 that the introduction of ether into the stomach produces a reflex 

 flow of pancreatic juice, but all attempts to discover the pat^h of 

 the nerve impulses failed until the recent work of Pawlow. The 

 reason of the failure of previous workers is that the pancreas is 

 remarkably sensitive to external conditions. If the pancreas is 

 cooled or wounded during the process of making the fistula,. or if 

 sensory nerves are excited, or if anaesthesia is deep, the gland 

 refuses to secrete. 



