404 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXV. 



sulphur. On digestion it is like proteid converted into peptone-like 

 substances, and is readily absorbed. Though it will replace in diet a 

 certain quantity of proteid, acting as what is called a ' proteid-sparing ' 

 food, it cannot altogether take the place of proteid as a food. Animals 

 fed on gelatin instead of proteid waste rapidly. 



Chondrin, the very similar substance obtained from hyaline 

 cartilage, is a mixture of gelatin with mucinoid materials. 



Blastin. This is the substance of which the yellow or elastic 

 fibres of connective-tissue are composed. It is a very insoluble 

 material. The sarcolemma of muscular fibres and certain basement 

 membranes are very similar. 



Keratin, or horny material, is the substance found in the surface 

 layers of the epidermis, in hairs, nails, hoofs, and horns. 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 PKOTAMINES. 



Protamines. These are basic substances which are combined 

 with nuclein in the heads of the spermatozoa of certain fishes (salmon, 

 sturgeon, etc.). 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 consequence the hexone bases ; the bases are 

 named lysine (C 6 H 14 N 2 2 ), arginine (C 6 H 14 N 4 2 ), and histidine 



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 subst inces 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 



