192 CONNECTIVE TISSUE. [BOOK i. 



while others are very large with a distinct double outline. 

 Whether small or large each fibre is a single fibre, not a bundle, 

 and cannot be split up, like a fibre or small bundle of the ordinary 

 matrix, into fibrillae ; in the larger fibres at least a sheath may be 

 distinguished from the substance of the fibre. Not only is their 

 course sharply curved unlike the gently sweeping outlines of the 

 gelatiniferous fibres, but they divide and anastomose freely, thus 

 forming networks of varying shape ; the gelatiniferous fibrillse on 

 the other hand never divide, and the bundles do not anastomose, 

 but simply interlace into a network. 



The number of these fibres occurring in connective tissue 

 varies much in different situations, and in some places, as for 

 instance in the lig amentum nuchas of certain animals, nearly the 

 whole tissue is composed of large fibres of this kind, having 

 in the mass a yellow colour, the ordinary gelatiniferous fibres 

 being reduced to a minimum. In such a situation a remarkable 

 physical character of these fibres is easily recognized ; they are in 

 a high degree extensible and elastic ; hence they are frequently 

 called elastic fibres ; from their yellowish colour they are sometimes 

 called yellow elastic fibres. The white gelatiniferous fibrillae on 

 the contrary possess very little extensibility or elasticity. 



When a portion of ligamentum nuchse is freed by prolonged 

 boiling from the remnant of gelatiniferous fibres mixed up with the 

 yellow elastic material, the latter is found on chemical treatment 

 to yield a substance called elastin, which very closely resembles 

 proteid matter in elementary composition except that it contains 

 no sulphur, and which yet probably differs widely from it in nature. 



Connective tissue then consists of a matrix of in extensible 

 inelastic white wavy gelatiniferous fibrillse, cemented into bundles 

 (the bundles being arranged, in loose connective tissue, in irregular 

 meshworks), with which are associated in varying abundance anasto- 

 mosing, curled, yellow elastic fibres, and among which are embedded 

 branched connective tissue corpuscles. Leucocytes of various kinds 

 are also found in the meshes or areolse of the mesh work. We may 

 now return to the structure of the blood vessels. 



107. Capillaries. A capillary is, as we said above, a tubular 

 passage hollowed out in connective tissue. Without special pre- 

 paration all that can be seen under the microscope is the outline 

 of the wall of the capillary, shewing under high powers a double 

 contour, and marked with oval nuclei which are lodged in the wall 

 at intervals and which project somewhat into the lumen or canal 

 of the vessel. When however the tissue containing the capillaries 

 is treated with a weak solution of silver nitrate, and after being 

 thoroughly washed is exposed to light, the wall of the capillary is 

 seen to be marked out by thin black lines into spindle-shaped 

 areas, dovetailing into each other, and so related to the nuclei in 

 the wall, that each nucleus occupies about the centre of an area. 

 From this and from other facts we conclude that the capillary 



