16 



HIGH SCHOOL ZOOLOGY. 



substance, in which case nuclei with a scant surrounding film of proto- 

 plasm are left embedded in a matrix which may be soft and jelly-like, 

 or converted into fibres, or stiff and homogeneous like cartilage, or hard 

 through incorporation with lime-salts like bone and tooth. 



III. Muscle-tissue. — The cellular character is to be seen in the simpler 

 kind which is formed of much elongated cells, whose protoplasm is 

 highly contractile, while the higher kind of muscle-tissue is that termed 



striated (from the appearance of the contractile substance of the fibres 

 under the microscope). In the latter, each fibre is a unit of higher 



rank than the simple muscle-cell, because it is the equivalent of several 



cells. 



IV. Nerve-tissue. — Two el^ients are distinguished, nerve-cells and 

 nerve-filjres. The cellular character of the former is always evident; 

 the latter are to be regarded as processes of these cells, each nerve- 

 fibre having for its core an 

 axis-cylinder c c 

 with the cell 

 which serves for 



tion, andisgeneraUyisolat- •^S!if''^&;^^^A/;,';'l'^>'^V,'*^ 

 ed by one or more sheaths. /-//E'^^/iffiL^.fy'^M/;// ^ikil'f»^^m^^^^ 



9. Histology of the 



Skin. — The microscopical 

 examination of a thin pre- 

 pared section of skin dis- 

 closes at once the two chief 

 component parts (Fig. 4). 

 Of these the horizontal fibres 

 of the corium are separated 

 from the epithelial layers by 

 a looser connective tissue, in 

 which pigment cells are 

 abundant and which pro- 

 jects into the papillae. Some 

 looser fatty connective tis- 

 sue may separate the hori- 

 zontal fibre-layers from the 



flesh. Filling up the inter- Fig. 4.-Diafrram of section of the Skin in the Cat- 

 spaces between the papillae ^^h. Ep, epithelium ; pap, papillary layer of 

 it, -i-u !• 1 11 i- *^^ coriiun, co ; sc, subcutaneous connective 

 9Xe the epithelial cells ot tissue, with nerves and blood-vessels. X50. 



