THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 541 



and the glue-producing intermediate tissue ; they are sharply 

 differentiated from their softer surroundings, and become adapted, 

 in consequence of their peculiar physical properties, to the as- 

 sumption of special functions. Cartilage serves in part to keep 

 canals open (cartilage of the larynx and bronchial tree), in part 

 for the protection of vital organs, around which they form a firm 

 envelope (cartilaginous cranium, capsule of the labyrinth, vertebral 

 canal, etc.), and in part for the support of structures projecting from 

 the surface of the body (cartilage of the limbs, branchial rays, etc.). 

 At the same time they afford firm points of attachment for the 

 masses of muscle imbedded in the mesenchyine, neighboring parts 

 of the muscles entering into firm union with them. In this manner 

 there has arisen through histological metamorphosis a differentiated 

 skeletal apparatus, which increases in complication in the same 

 proportion as it acquires more manifold relations with the muscu- 

 lature. 



Cartilaginous and connective tissues, finally, are capable of a 

 further histological metamorphosis, since the last form of sustenta- 

 tive substance, osseous tissue, is developed from them in connection 

 with the secretion of salts of lime. There are therefore bones that 

 have arisen from a cartilaginous matrix ami others from one of con- 

 nective tissue. With the appearance of bone, the skeletal apparatus 

 of Vertebrates has reached its highest perfection. 



Even if the rnesenchyme has by these processes experienced an 

 extraordinarily high degree of differentiation and a great diversity 

 of form, the histological processes of differentiation which take place 

 in it are nevertheless not yet exhausted. In the gelatinous or 

 connective-tissue matrix canals and spaces arise in which blood and 

 lymph move in accomplishing their function of intermediating in 

 the metastasis of the organism, not only conveying the nutritive 

 fluids to the individual organs, but also conducting away both the 

 substances which owing to the chemical processes in the tissues 



-have become useless and the superfluous fluids. Out of these 

 first beginnings arises a very complicated organic apparatus. The 

 larger cavities constitute arteries and veins, and acquire peculiarly 

 constructed thick walls, provided with non-stria te muscle-cells 

 and elastic fibres, in which three different layers can be dis- 

 tinguished as tunica intinia, media, and adventitia. A small part 

 of the blood-passages, especially distinguished by an abundance of 

 muscle-cells, is converted into a propulsive apparatus for the fluid 



-the heart. The elementary corpuscles that circulate in the 



