4 GENERAL CONSIDERATIONS. * 



the majority of Fishes. These cells have an envelope, but they do not have 

 a nucleus in Mammalia ; 



2. Leucocytes, white corpuscles, or lymph ceZZs, which float in the blood, lymph, 

 chyle, and the connective tissue interspaces. These cells are susceptible of amse- 

 boid movements, and their fundamental form is spherical ; though they are often 

 irregular in shape, owing to the contractile prolongations they throw out, called 

 pseupodes. They may have one or more irregular nuclei ; 



3. Connective cells, which are flat, nucleated, and irregular ; they are applied 

 to the fasciculi of connective fibres ; 



4. Adipose cells, lying in the connective tissue and filled with fat, which has 

 pushed the protoplasm and nucleus towards the envelope ; 



5. Medullary cells, with a budding nucleus or multiple nuclei (myeloplaxes 

 and medullo-cells), forming the principal elements of the marrow of bones ; 



6. Ckirtilage cells, which have no envelope, and which, single or associated 

 with a variable intermediate substance, form cartilages ; 



7. Bone cells, lodged in a space remarkable for its elliptical shape and the 

 numerous prolongations on its margin. The cells are nucleated, have no envelope, 

 and secrete the fundamental sohd substance of the bony tissue ; 



8. Contractile cells, which constitute the basis of muscular tissue. They are 

 fusiform and nucleated ; 



9. Nerve cells, met with in the cerebro-spinal centres and the ganglia of the 

 cerebro-spinal and sympathetic systems. They are provided with one, two, or 

 more prolongations called poles ; hence they are uni-, hi-, or multipolar. 



10. Epithelial cells, found on the surface of tegumentary membranes, or laid 

 over the interior of more or less divert iculated cavities (glandular cavities or glands). 

 The epithelial element is lamellar or polyhedric, cylindrical, calcif orm, vibratile, etc, 



11. Endothelial cells, always lamellar, and lining serous cavities and vascular 

 canals. 



Fibres. — A fibre is an elongated element of variable dimensions and com- 

 position. Thus, it may be very fine (connective fibre), or thick and limited by 

 two borders more or less apart {muscular fibre) ; it is homogeneous throughout 

 {elastic fibre), or has a contents and a distinct envelope {ne^'ve fibre). The vitality 

 of fibres is not comparable with that of cells. Once formed, they are only 

 nourished, for it is not yet definitely ascertained whether they can multiply. 



In the animal economy there are four kinds of fibres — 



1. Connective fibre, an extremely fine element, but in which, nevertheless, 

 two borders can be distinguished if it be examined by a power of from 800 to 

 1000 diameters. The fibres form fasciculi in the connective tissue, or are distri- 

 buted in the midst of a fundamental substance — as in the fibro-cartilages. The 

 connective fibre constitutes the most solid organs of the animal economy — the 

 ligaments, tendons, etc. 



2. Elastic fibre, found closely packed in certain organs, such as the cervical 

 ligament and abdominal tunic, or forming a network in the midst of connective 

 tissue ; it is even found in the framework of bones. 



3. Muscular fibre, smooth or striped (striated), which has the property of 

 contracting under the influence of stimuli. Smooth (or unstriped), is found 

 in a large number of viscera ; striped is more especially in the domain of the 

 locomotory apparatus. 



4. Nerve fibre, very remarkable for its continuous axis-cylinder. It is foimd 

 in the nerve centres, cerebro-spinal nerves, and great sympathetic system. 



