4O2 THE CHEMICAL COMPOSITION OF THE BODY. [CH. xxv. 



It is very insoluble, and chiefly differs from proteids in its high 

 percentage of sulphur. A similar substance, called neurokeratin, 

 is found in neuroglia and nerve fibres. In this connection it is 

 interesting to note that the epidermis and the nervous system are 

 both formed from the same layer of the embryo the epiblast. 



Chitin and similar substances found in the exoskeleton of 

 many invertebrates. 



D. THE PROTAMINES. 



Protamines. These are basic substances which are combined 

 with nuclein in the heads of the spermatozoa of certain fishes 

 (salmon, sturgeon, &c.). They resemble proteids in many of 

 their characters ; e.g., they give Piotrowski's reaction and some of 

 the other tests for proteids. They are regarded by Kossel as 

 the simplest proteids. By decomposition in various ways they 

 yield bases containing six atoms of carbon, and called in con- 

 sequence the hexone bases ; the bases are named lysine, arginine, 

 and histidine. The following equation represents the decom- 

 position of the protamine prepared from salmon roe : 



C3oH 57 N 17 06 + 4 H 2 = C 6 H 9 N 3 2 + 3 C 6 H 14 N 4 2 + C 6 H 14 N 2 2 



[Protamine] [Histidine] [Arginine] [Lysine] 



The more complex proteids and albuminoids yield these bases 

 also ; therefore Kossel considers that all these substances contain 

 a protamine nucleus. The more complex proteids, however, 

 yield many other products of decomposition in addition to these 

 bases, such as leucine and tyrosine. 



The Polarimeter. 



This instrument is one by means of which the action of various substances 

 on the plane of polarised light can be observed and measured. 



Most of the carbohydrates are dextro-rotatory. 



All the proteids are levo-rotatory. 



There are many varieties of the instrument ; these can only be properly 

 studied in a practical class, and all one can do here is to state briefly 

 the principles on which they are constructed. 



Suppose one is shooting arrows at a fence made up of narrow vertical 

 palings : suppose also that the arrows are flat like the laths of a Venetian 

 blind. If the arrows are shot vertically they will pass easily through 

 the gaps between the palings, but if they are shot horizontally they will be 

 unable to pass through at all. This rough illustration will help us in under- 

 standing what is meant by polarised light. Ordinary light is produced by 

 the undulations of aether occurring in all directions at right angles to the 

 path of propagation of the wave. Polarised light is produced by undulations 

 in one plane only ; we may compare it to our flat arrows. 



In a polarimeter, there is at one end of the instrument a Nicol's prism, 

 which is made of Iceland spar. This polarises the light which passes 

 through it ; it is called the polariser. At the other end of the instrument 



