HISTOLOGY OF HIGHER ANIMALS 



57 



it is to a greater or less extent supplemented or even replaced by 

 bone. It consists mainly of a tough, translucent matrix, or 

 intercellular substance, which is formed as a secretion by the 

 cartilage cells and in which the latter are imbedded at wide 

 intervals (Fig. 20). In this respect it differs greatly from the 

 epidermis, in which the cells lie close together and little or no 

 intercellular substance is developed. The cartilage grows by 

 repeated division of the cells which it contains and the secre- 

 tion of additional intercellular matrix between 

 them. The frequent arrangement of the 

 nucleated cells in pairs, as shown in the 

 illustration, is an indication of recent cell- 

 division. Bone is a more complex tissue than 

 cartilage and is further strengthened and 

 hardened by the deposition of calcareous 

 salts, chiefly phosphate of lime, in the matrix. 



Muscular tissue is specialized in a totally 

 different direction from any of the foregoing. 

 Its function is to contract, and by so doing 

 to bring about the various movements of 

 which the higher animals are capable. There 

 are two very distinct kinds of muscular tissue, 

 the one comparatively simple and the other 

 much more complex in structure. The former, 

 which is known as unstriped muscle (Fig. 21), 

 consists of greatly elongated cells, the muscle- 

 fibres, associated in sheets or bundles. Each 

 has a centrally placed nucleus and its cellular 

 nature is at once obvious. The wall of the 

 alimentary canal, outside its lining epithelium, 

 is composed chiefly of muscle-fibres of this 

 kind. Their rhythmical and co-ordinated contraction causes 

 the characteristic peristaltic movement whereby the onward 

 passage of the food is secured. These and similar movements 

 effected in other organs by the action of unstriped muscular 

 tissue take place quite independently of the will whence the 

 term " involuntary " is often applied to this type of muscle. 



Striped or striated muscular tissue is usually under the control 

 of the will, and is hence often spoken of as " voluntary," but it is 

 found in the higher animals wherever very sharp, precise move- 

 ments are required, as for example in the walls of the heart, the 



FiG.21. Uustriped 

 Muscle Fibres 

 from the Wall of 

 the Eabbit's In- 

 testine, X 300. 



nu., nuclei. 



