36 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



cartilage or elastic cartilage, in which a dense net-work of elastic fibres 

 occupies the intercellular matrix). Most cartilages, except on articulating 

 surfaces, are covered by a fibrous membrane, the perichondrium, supplied 

 with blood-vessels, lymphatics, and nerves. 



Bone is surrounded by a fibrous membrane, the periosteum, with an 

 inner layer of oblong nucleated cells, which, from the fact that the bone 

 is developed from them, are termed osteoblasts. Similar cells are also 

 found in the marrow of bones. 



The organs of the animal body are always composed of several 

 tissues ; they may be of three classes, in each one of which some one 

 tissue is especially prominent in function : — 



1. Organs whose chief function depends upon tissues of the first 

 class (cells without intercellular substance). 



a. Glands, which always in addition to the epithelium contain nerves 

 and blood-vessels. The epithelium is the essential part of glands, skin, 

 mucous and serous membranes. 



b. Muscles. In these are found muscle-cells as the functionally 

 prominent tissues, though associated with them are connective tissue, 

 blood-vessels, and nerves. 



2. Organs whose chief function is manifested through tissues of the 

 second class. 



a. Compound vessels, arteries, veins, and lymph-vessels. Although 

 the capillaries are formed by union of cells, in larger trunks this mode 

 of formation only applies to the endothelium, on which the other tissues 

 — elastic tissue, connective tissue, and pale muscular fibres — subsequently 

 develop in layers. 



b. The organs of the nervous system. Nerve-cells and nerve-fibres, 

 formed by union end to end of cells, here form the essential tissues, 

 though blood-vessels and connective tissue are associated with them. 



3. Organs whose function is clue to intercellular substance. The bony 

 skeleton is the only representative of this group, and the modification 

 of connective tissue known as bone, or its antecedent cartilage, is the 

 characteristic tissue; as secondary tissues, connective tissue and blood- 

 vessels and nerves, representatives of the second class, are also met with. 



For the mode of development of the compound organs in the 

 embryo the reader must be referred to text-books on anatomy or 

 embryology. 



