THE FUNDAMENTAL TISSUES. 25 



are all examples of epithelia. The epidermis is ectoderm; the lining of 

 the intestine is entoderm; that of the blood vessels, called endothelium, is 

 from the angioblast; the peritoneal epithelium (mesothelium) is part of 

 the splanchnic and somatic layers of mesoderm; and the joint cavities are 

 lined by flattened mesenchymal cells, the cavity being, as it were, a large 

 intercellular space. Thus epithelia are derived from all the germ layers. 



Mesenchyma is a non-epithelial portion of the mesoderm, which 

 has just been described as consisting of branched cells, the protoplasmic 

 processes of which form a continuous network. In its meshes is a clear 

 intercellular fluid. Mesenchyma is essentially a tissue of the embryo. 

 In the adult it is represented by connective tissue, bone, and other deriva- 

 tives which preserve certain of the characteristics of mesenchyma. 



Three other forms of tissue depart so far from the epithelial and 

 mesenchymal types that they are naturally placed by themselves. These 

 are muscle, nerve, and vascular tissue. Muscle tissue exists in three forms, 

 of which the smooth and cardiac varieties are derived from mesenchyma, 

 and the striated (voluntary) muscles from the mesodermic segments. The 

 epithelial character of the latter is lost. Nerve tissue is ectodermal, con- 

 sisting of an epithelial tube which later becomes essentially non-epithelial, 

 and of detached masses of cells which send processes to all parts of the 

 body, forming the nerves. These are never epithelial. Vascular tissue 

 includes the blood and the lymph, which are of obscure origin, perhaps 

 mesenchymal; also the endothelium which lines the vessels, provided that 

 the blood and the endothelium have a common origin. It will be con- 

 venient to describe the entire blood vessels and lymphatic vessels in connec- 

 tion with their contents. 



In the following pages the several tissues will be considered in the 

 order above outlined. In connection with them, certain organs may be 

 examined. An organ is a, more or less independent portion of the body, 

 having its own blood, lymphatic and nerve supply, and connective tissue 

 framework, together with its characteristic essential cells. Thus an organ 

 should consist of several tissues. The pancreas or lungs are obviously 

 organs. An individual muscle or a particular bone has a connective 

 tissue framework or covering,.blood vessels and nerves, besides its essential 

 substance. Thus it is an organ. Even a blood vessel of ordinary size 

 comes within the definition. The organs which are of wide occurrence 

 like the bones, muscles, tendons, nerves and vessels, may be described 

 with their essential tissues. The more complex organs are reserved for 

 the latter section entitled "Special Histology." 



Before presenting in summary form the derivatives of the germ layers 

 it should be noted that the ectoderm becomes continuous with the ento- 



