52 



CONNECTIVE TISSUE. 



Fis. 



doubted, that in both the coloured and white races, the colouring matter 

 of the skin is the same in its essential nature 

 us that of the choroid. In Albino individuals,, 

 both negro and European, in whom the black 

 matter of the choroid is wanting, the cuticle and 

 the hair are colourless also. 



In some situations the pigment is met with 

 in enlarged and irregularly branched corpuscles 

 which belong to the connective tissue. Such rami- 

 fied cells are very common in many animals. 

 In the human body cells of this description are 

 found in the dark tissue on the outer surface of 

 the choroid coat, lamina fusca (fig. 27, a a), 

 and on the pia mater covering the upper part of 

 the spinal cord. The condition of the pigment 

 in the hairs will be afterwards noticed. 



When the cuticle of the negro is removed by 

 means of a blister, it is renewed again of its 

 original dark hue ; but if the skin be destroyed 

 to any considerable depth, as by a severe burn, 

 the resulting scar remains long white, though it 

 at length acquires a dark colour. 



i'ig. -]'. — IvAMiFiED Cells, 

 tkoji the tissue of the 

 Choroid Coat of the 

 Eye ; magnified 350 

 DIAMETERS (Kolliker). 



a, cells with pigment ; 

 h, colourless fusiform cells. 



Composition. — Examined chemically, the black matter is found to be in- 

 soluble in cold and hot water, alcohol, ether, fixed and volatile oils, acetic and 

 diluted mineral acids. The pigment of the bullock's eye, when purified by boiling 

 in alcohol and ether, was found by Scherer to consist of .58"672 carbon, 5'i}(32 

 hydrogen, lo-768 nitrogen, and 2r.5y8 oxygen ; its proportion of carbon is thus 

 very large. Preceding chemists had obtained from its ashes oxide of iron, 

 chloride of sodium, lime, and phosphate of lime. 



Uses. — In the eye the black matter seems obviously intended to absorb re- 

 dundant light, and accordingly its absence in Albinos is attended with a difficulty 

 of bearing a light of considerable brightness. Its uses in other situations are not. 

 so apparent. The pigment of the cuticle, it has been supposed, may screen the 

 subjacent cutis from the pungency of the sun's rays, but in many animals the 

 pigment is not only employed to variegate the surface of the body, but attaches 

 itself to deep-seated parts. Thus, in the frog the branches and twigs of the 

 blood-vessels are speckled over with it, and in many fish it imparts a black colour 

 to the peritoneum and other internal membranes. 



CONlSrECTIVE TISSUE. 



This substance consists of fibres of two kinds, more or less amorphous 

 matter, and peculiar corpuscles. By means of its fibres it serves in the 

 animal body as a bond of connection of different parts ; also as a 

 covering or investment to different organs, not only protecting them 

 outwardly, but, in many cases entering into their structure and con- 

 necting and supporting their component parts. The corpuscles, on the 

 other hand, are destined for other than mechanical purposes ; they 

 appear to be essentially concerned in the nutrition and reparation of 

 tissues. 



Three principal modifications or varieties of connective tissue have 

 long been recognised, consisting of the same structural elements, but in 

 widely different proportions, and thereby exhibiting a diflFerencein their 

 grosser or more obvious characters and physical properties. They are 



