426 MAMMALIA— PHYSIOLOGY 



from epithelium derived from the primitive mesoderm or in some 

 cases from ectoderm or endoderm. When the sexual cells develop 

 (somewhat as glands form) they frequently starve the adjacent cells. 



Connective tissues are derived from the mesoderm. Mucous 

 tissue is found in embryonic animals. The umbilical cord and the 

 vitreous humor of the eye are mucous tissue. Reticular tissue is 

 fibrous connective tissue in which the inter-cellular substance has 

 disappeared. It is found in adenoid tissue and in lymph glands, 

 in the spleen, and in the mucous membrane of the intestinal canal. 

 Fibrous tissue consists of areolar, tendinous and elastic tissue. 

 {a) Areolar tissue consists of bundles of white fibers with a few 

 fibrils of elastic tissue. The inter-spaces are filled with lymph. 

 {h) Tendons and ligaments consist of dense white fibrous tissue. (<r) 

 Elastic tissue consists of yellow elastic fibers, frequently found in 

 bundles. 



Adipose tissue consists of a matrix of areolar tissue containing 

 cells, the nuclei of which are a-central. 



Cartilage may be hyaline, elastic or fibro-cartilage. {a) Hyaline 

 cartilage is jelly-like and found in embryonic bones and at the ends 

 of bones. It has a homogeneous matrix and prominent cells. {]?) 

 Elastic cartilage is found in the lobe of the ear, in the epiglottis and 

 in the Eustachian tube, {c) Fibro-cartilage is found in inter- 

 vertebral substance and in the sesamoid bones. It consists of 

 groups of slightly flattened nucleated cells enclosed in capsules and 

 scattered among fibers. 



Bone. — Bony tissue may be either compact and dense like ivory 

 or spongy and cancellous with many interstices. The outer layer 

 is called t\\Q periosteum. In true bone we find fine fibers in a calcified 

 ground substance with branched cells, the bone corpuscles., lying in 

 cell spaces called lacunae situated in folds or lamellae around a cavity 

 called the Haversian canal. Ramifying passages which contain cell 

 processes are called canaliculi and radiate in all directions from the 

 lacunae. 



Muscle substance is a specialized contractile tissue which is the 

 agent of active movement. It is distinguished from undifferen- 

 tiated protoplasm since it contracts in one direction. In inverte- 

 brates such as protozoa, sponges, and coelenterates we find primitive 

 contractile fibrils, but in worms, molluscs and arthropods, muscular 

 tissue is highly developed. Non-striated or smooth 7nuscle cells 

 are involuntary. The cells are long, spindle shaped, and somewhat 



