STRUCTURAL ELEMENTS OF THE TISSUES. 67 



epithelium is covered with minute soft thread-like appendages named 

 cilia, attached to the free ends of the cells, and is hence called ciliated 

 columnar epithelium, b. By some the soft homogeneous substance 

 composing these cilia is considered to be sarcodous. In the windpipe, 

 this sort of epithelium is stratified or has many layers of cells, d, the 

 superficial ones only being columnar and provided with cilia. In 

 certain cavities in the interior of the brain, the epithelium is said by 

 the best authorities (though it is doubted by some observers) to be 

 ciliated, but the cells are flattened, not columnar. In some animals, 

 too, a spheroidal tubular epithelium is met with, having cilia upon it, 

 as in the roof of the frog's mouth. 



There are some special tissues, or modifications of tissue, such as 

 the teeth, nails, and hair, the humors and other parts of the eye, and 

 certain parts of the ear and nose, which will be described hereafter. 

 The structure of the different secreting glands will be considered in 

 the chapters treating of their respective functions ; and so, likewise, 

 will the structure of those organs which, from their general resem- 

 blance to the secreting glands, have also been called glands, viz., the 

 ductless or vascular glands, or blood-glands, which include the spleen, 

 supra-renal bodies, thyroid body, and thymus gland, and also the 

 closed saccular glands of the alimentary canal, represented by those of 

 the tongue and tonsils, by the solitary glands of the stomach and in- 

 testines, and by the so-called Peyer's glands found in the small intes- 

 tine only. 



General View of the Structural Elements of the Tissues. 



If now we glance generally at the numerous elementary microscopic 

 constituents of the tissues, we find that, however, varied they may be, 

 they are all referable to one or other of the following terms : interme- 

 diate connecting substance, named blastema or matrix, crystals, proto- 

 plasm, granules, homogeneous or structureless membrane, vesicles, 

 nuclei, nucleated cells, simple fibres, nucleated fibres, compound fibres, 

 and tubes. The blastema or matrix may be either fluid, as in the case 

 of the liquor sanguinis of the blood, or softish, as in the moist tissues ; 

 either abundant, as in the soft forms of connective tissue, scanty, as in 

 some epithelia, or absent, as in a peculiar reticular kind of connective 

 tissue, found in the lymphatic glands and elsewhere ; or the matrix 

 may be dry and scanty, as in the cuticle, or abundant and fibrous, as 

 in the connective tissues, homogeneous, as in pure cartilage, or fibrous 

 and calcified, as in bone. Crystals are rare except in disease. Pro- 

 toplasm is the soft, minutely granular substance, so universal in both 

 the animal and vegetable kingdoms, and the earliest recognizable form 

 of organic matter. Separate elementary granules are present in the 

 chyle, the blood, the brain-substance, the pigment- tissue, and else- 

 where. Elementary vesicles, consisting of fatty matter, exist in the 

 chyle, the blood, and the milk. Free nuclei occur in the cerebellum, 

 and are also represented in the lymph corpuscles. Structureless mem- 

 brane forms the basement-membrane of the mucous tissues and glands ; 

 also the walls of certain nucleated cells, and the coats of the finest 



