34 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the small spherical lymphoid cells which result are carried to the circu- 

 lation to become white blood-corpuscles. 



Glandular tissue is also epithelial in nature. Cells are the essential 

 secreting organs of glands, though numerous other tissues enter into 

 their composition. Such cells are usually rounded or polygonal, and are 

 soft in consistence. Frequently the cells rapidly partially break down 

 and are carried off as constituents of the secretion, as colostrum cells, 

 or mucoid corpuscles (moulting); or the destruction of the cell form may 

 bo complete and the constituents of the cell enter the secretion in the 

 form of a solution. 



Muscular tissue forms the third of the group. In the muscles the 

 muscular cells are always closely associated with connective tissue. 

 Such cells may be of two different kinds, different in structure and in 

 function, — the striped and unstriped cells. Muscle-cells are contractile, 

 like amoeboid cells, but contractility is only possible in one definite direc- 

 tion, that of their long axis; muscles, therefore, become shorter and 

 thicker during contraction. 



2. To the second group of tissues formed by union of cells belong 

 two tissues in which, in their development, the cells have become greatly 

 elongated, and after absorption of the cell-wall become converted into 

 fibres or tubes ; these are the nerves and capillaries. 



In the nervous tissues certain cells retain their primitive character, 

 the nerve-cells, and only nerve-fibres properly belong to this group of 

 tissues. Nevertheless the nerve-cells are in continuity with the nerve- 

 fibres. 



The capillaries in a similar manner originate from the arrangement 

 of nucleated, spindle-shaped cells in rows, which become hollowed out 

 through the formation of vacuoles. The formative cells manifest a ten- 

 dency to send out sprouts which connect with other forming or mature 

 capillaries by which the net-work of capillaries is formed. Lymphatic 

 capillaries are developed in the same way as the blood-vessels. 



3. Tissues formed, by excretions from cells, forming the third group, 

 may also be called intercellular tissues. Connective tissue is a type of 

 this class. Connective tissues serve to bind together all the organs of 

 the body ; they exist in various forms, which are to a certain extent 

 mutually convertible; they all yield allied chemical products. 



Connective tissue originates in spherical cells, with very soft proto- 

 plasmic nucleated contents, which ultimately may form a perfectly homo- 

 geneous, laminated, or fibrous intercellular substance. 



The cells themselves may exist in various different forms, either as 

 tendon-cells, when they are flattened, oblong masses of protoplasm 

 arranged in rows, with round nuclei lying in bundles of fibrils of white 

 connective tissue ; as branched cells, found in the cornea, serous mem- 



