ANIMAL TISSUES. 11 



different layers : the finest vessels (called Capillaries) of a 

 single uniform membrane, without fibres or other tissue, 

 but in which lie round or oblong microscopic corpuscles 

 (cell-nuclei). In the larger blood-vessels, and also in the 

 lymphatics, the innermost coat is formed of cells, which, 

 as in the Cuticle, lie side by side, pavement-fashion. This 

 coat is covered by others which present fibres partly longi- 

 tudinal, partly circular. Then comes the most external 

 layer of conjunctive tissue, which connects the vessels with 

 the neighbouring parts. 



IV. Nervous Tissue (tela nervea). To this belongs in the higher 

 animals, the brain, the spinal cord, the ganglia and the 

 nerves: inferior animals have only nerves and ganglia, 

 which last take the place of the central parts of the nervous 

 system. The chemical constituents of this tissue are 

 Albumen and a species of Fat containing Phosphorus. 

 The nerve-stems and the bundles of which they consist, 

 are surrounded with coats of conjunctive tissue, called 

 Neurilema : dilute muriatic acid dissolves the neurilema : 

 alkaline solutions, on the contrary, cause the nervous 

 medulla to disappear, the neurilema remaining alone. The 

 nerves consist of fine threads, which neither subdivide, nor 

 anastomose with each other. They are of very unequal 

 thickness, ^ ... to j~ millim. and less, especially in the 

 nerves of sense. Besides these threads there are found 

 corpuscles with nuclei : these present themselves in the 

 ganglia and in the grey substance of the brain and spinal 

 cord. These ganglion-corpuscles are very dissimilar in 

 form and size, mostly ^ . . . ^ millim. 



V. Horny Tissue (tela cornea). The parts consisting of this 

 tissue have neither blood-vessels nor nerves. Cuticle, nails, 

 hair, feathers, horns and scales belong hereto. They lie on 

 the surface of the body, whilst a covering (epithelium} , resem- 

 bling Cuticle, lines the inner surface of the mucous membranes 

 (as of the stomach) and also of the internal closed cavities 

 and sacs, as well as of the vessels (see above, III. Vase. 

 Tis.). The Cuticle, or Epidermis, consists of microscopic flat 

 cells joining on to each other like a pavement, and of which 

 each contains a nucleus. Water swells up the epidermis, 



