48 



ESSENTIALS OF BIOLOGY 



13. ON ORGANIC TISSUES 



Whatever they are, the bodily organs are aggregates of tissues. 

 An organ is a definite tectonic arrangement of many kinds of 

 tissues. Thus the eye of a vertebrate is buih up of skeletal tissue 

 (the sclerotic), nervous tissue (the retina), vascular tissue (the 

 blood-vessels), etc. Tissues are aggregates of differentiated cells. 



Fig. 14. 



I, Bone tissue ; 2, cartilage ; 3, the shell, or carapace of an invertebrate ; 4, the skin of 

 a fish, showing the scales ; 5, striated muscle-fibres ; 6, muscle-fibres from the heart of a 

 vertebrate ; 7, an involuntary (unstriated) muscle-fibre ; 8, a muscle-fibre from a Trematode. 



Skeletal Tissues. It is customary to speak of the " Living " cells 

 (such as bone-corpuscles) that secrete the matrices and the non- 

 living substance of the latter. This distinction is not always clear, 

 for it may not be possible to decide in what way a thick cell-wall 

 differs from an inter-cellular matrix, and it may not be possible 

 to decide w^hether or not a cell- wall is any less alive than other 

 parts of the cytoplasm. Cartilage has cells imbedded in it. 

 Bone is a matrix containing cells and cell-filaments. In all cases 

 a skeletal tissue, bone, cartilage, shell, etc., has a framework of 

 cells that secrete the substance of the matrix. 



Muscular tissue. A muscle is an organ ^^hich has in it muscle- 

 cells, nerves, blood-vessels, lymph vessels and connective tissue. 

 The characteristic and most prevalent tissue consists of arranged 

 muscle-cells. In the muscle these cells are compacted together 

 by their own external membranes and by interstitial connective 

 tissue. It is, of course, the co-ordinated contractions of all, or 



