PHYSICAL PROPEKTIES OF TISSUES. 69 



ing through them into two rays ; hence, when an object is seen through 

 such a crystal it appears double, the bifurcation of the ray of light being 

 spoken of as double refraction. 



Many of the animal tissues are doubly refractive, though this prop- 

 erty is weaker in fresh tissue than after drying. Double refraction is 

 only faintly developed in connective tissue, especially in its youngest 

 stages. Elastic tissue is more highly doubly refractive, as are also car- 

 tilage, bone, nerves, muscles, nails, and hair. 



Double refraction permits the recognition of the molecular structure 

 of organized tissues. A body whose molecules in all directions are 

 arranged in the same manner produces only single refraction ; one whose 

 molecules are arranged in different directions in different proportions 

 produces double refraction, i.e., splits the ray of light into two rays, 

 which are polarized perpendicularly to one another, and whose vibrations 

 are therefore in two planes perpendicular to one another. Simple glass 

 is a single refractive medium, but if compressed or stretched in one 

 direction it becomes doubly refractive. The double refractive body can 

 either, as in the last example, refract the ray more or less in one direction 

 than in the direction perpendicular to it, or the light can be transmitted 

 in three perpendicular directions with different velocities. In the inor- 

 ganic world crystals furnish examples of all three cases. 



Crystals of the regular system (tesseral) are isotropic (singly 

 refractive). In tetragonal and hexagonal forms, which possess an unequal 

 axis and two or three perpendicular equal axes, the refraction is either 

 greater (positive) or less (negative) in the direction of the unequal axis, 

 and such bodies are said to have a single optic axis. Other crystalline 

 forms have three axes, characterized by the transmission of light with 

 different velocities. They have two optic axes not coinciding with the 

 axes of crystallization. 



In organized bodies all of the above cases are also met with. Most 

 mature tissues are doubly refractive. The optic characteristics are not, 

 however, changed by pressure or stretching. 



We must conclude from this that the doubly-refractive tissue-mole- 

 cule is suspended in a singly-refractive medium, and that this molecule 

 is unaffected in pressure or stretching just as it remains unaffected in 

 imbibition. Organic tissues are therefore analogous to crystals in their 

 molecular arrangement ; and this view is strengthened by the fact that 

 many organic substances which are apparently anything but crystalline 

 in their structure, such as albumen, gluten, and chondrin, possess the 

 •power of rotating the plane of polarized light. 



The most important examples of double refractive power are seen in 

 the muscles and nerves. These will be considered under their special 

 headings. 



