68 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



All the connective tissues are originally flexible and extensible ; 

 these properties become greatly modified in the subsequent development 

 of the tissues of this group. Thus, in cartilage and bone, extensibility 

 has very largely disappeared, especially in the latter, but they are of high 

 elasticit}'. In dense fibrous tissue, such as aponeuroses, flexibility re- 

 mains, but extensibility has become greatly reduced ; hence the intense 

 pain produced in inflammation below such tissues ; for being inextensible 

 swelling is restrained, and the pressure produced by the products of in- 

 flammation on the nerve-endings is greatly increased. 



3. OPTICAL CHARACTERISTICS OF TISSUES (Wundt). (a) Ee fraction. 

 All organic tissues possess a higher refractive index than water. By this 

 is meant that when a ray of light passes obliquely out of one medium 

 into another of different density, it is bent out of its path in a straight 

 line at the surface of separation of the two media, the ratio between the 

 angle of incidence and the angle of refraction being the index of refrac- 

 tion. Though comparative measurements of the different tissues have not 

 been made, we can recognize the difference by the sharpness of outline 

 in microscopic examination. Thus, cell-wall, nucleus, and nucleolus are 

 recognized by their difference in refractive powers. When two tissues 

 have the same refractive power they cannot be distinguished by the eye, 

 and if no refractive power is possessed they are homogeneous. 



Fat, elastic tissue, and horn have the highest refractive power. 

 Watery solutions, as in the vacuoles of plant-cells and in secretions, have 

 least refractive power. Albuminous matters, gelatin-giving intercellular 

 substance, and mucin have about the same refractive index. 



(b) Power of Absorbing Color*. In very thin sections most vegetable 

 and animal tissues appear colorless. In thick sections, when examined 

 by transmitted light, the different colors are absorbed in different degrees. 

 Yegetable tissues absorb the most refractive rays ; therefore, in sections 

 of increasing thickness they appear at first yellow and then red. The 

 same rule applies to animal tissues, even when freed from blood, e.g., 

 epithelium and cartilage. Many tissues owe their color to deposits in 

 them of special coloring matters. When this is intense many rays of 

 light are entirely extinguished, and in the spectra of such bodies portions 

 of the spectrum are either entirety absent or dark absorption-bands 

 appear in different parts of the spectrum. 



The points of occurrence of these absorption-bands are definite and 

 characteristic for each different substance. The spectra of certain bodies 

 of physiological importance, such as the blood, biliary coloring matters, 

 etc., will be referred to under their appropriate headings in the sections 

 on Special Physiology. 



(c) Double Refraction. A large number of bodies of crystalline 

 structure have the property of splitting a single incident ray of light pass- 



