38 HISTOLOGY. 



4. Cartilaginous tissue. 



5. Osseous tissue. 



6. Petrous tissue. 



Compound Tissues: 



, ,/r , /., ,. (1. Striped muscular fibre. 



1. Muscular fibrous tissue, ' 



muscular fibre _ 

 2. Nerve-fibrous tissue. 



Binary Tissues, formed of two simple tissues: 



1. Areolar tissue, constituted by the white and yellow fibrous 



tissues intermixed. 



2. Fibro- cartilage, constituted by cartilage and white fibrous 



tissue intermixed. 



A certain difference exists between the simple corpuscles and nucleated 

 cells. The corpuscles of the blood, for instance (in the mammalia), are 

 destitute of a nucleus, and are persistent ; whereas a nucleated cell is 

 always in a state of progression, either producing a fluid or undergoing a 

 transformation. 



The 1st variety of epithelium is found where there is a necessity for 

 transparency, as in the capsule of the lens and the posterior layer of the 

 cornea. The membrane in this case may be produced by the develop- 

 ment of a very large cell and the collapse of its walls, so as to cover the 

 whole area, thus constituting a duplicate lamina. 



The 2d variety is formed of hexagonal plates, adhering to each other 

 and containing a form of carbon ; it is found in the eye, in the lung, and 

 mixed with the cuticle and hair of the negro. 



The 3d variety constitutes the free surface of many membranes, as the 

 skin, the mucous and serous ; it may be found in a single lamina, forming 

 a pavement of nucleated cells, flattened and adhering by their edges ; or it 

 may form superimposed laminae, the exterior of which is constantly peeling 

 off, and the interior as constantly reproducing new cells to keep up the 

 covering. This is the case in the cuticle, and in the mucous membranes 

 of the mouth, oesophagus, rectum, &c. 



The 4th variety exists in mucous membranes, and consists of conoidal 

 nucleated cells, very firmly paved together. 



The 5th variety differs only from the last in having the base furnished 

 \*ith vibratile cilise, which diffuse the secretion, moistening the surface by 

 tneir constant motion. This kind of epithelium occurs in mucous and 

 serous membranes, where the surfaces cannot -come together and rub 

 against each other, as in the ventricles of the brain, the trachea, &c. 



Nails, hairs, and horny excrescences, are manifestly modifications of 

 epithelium, and are hence included in the enumeration as constituting tht 

 sixth variety of that tissue. 



The White fibrous tissue exists in ligaments and tendons, and consti- 

 tates the principal part of the derm or cutis vera. It is inelastic and in- 

 extensible. 



The Yellow fibrous tissue is found in the ligamenta fiava of the spine, 

 in the middle coat of the arterial system, and in the skin, mixed with the 

 white fibrous element. 



The Cartilaginous tissue constitutes the cartilage of the ribs, and the 



