HISTOLOGY AND CYTOLOGY 357 



The muscular tissues, as we have seen, are of two kinds, exhibiting 

 a fundamental difference. In the first place we have the varieties 

 in which the cellular structure is retained as in the involuntary and 

 cardiac muscle. In the second place there is the voluntary muscular 

 tissue which, while commencing in the embryo as strands of meso- 

 dermal cells, ends as long complicated fibres without any trace of 

 cellular structure, but in the formation of each of which a number of 

 cells took part. All muscular tissue is characterised by exhibiting 

 the power of contractility to a marked degree. 



The nervous tissue is composed of highly modified and very 

 characteristic cellular elements, the neurons, and much of the total 

 bulk of the nervous tissue in the body is constituted by the processes 

 of these cells and the protective medullary sheath they have 

 developed around them. 



There is, however, another and entirely different way of 

 classifying the body tissues in animals, and that is according to their 

 derivation. As we shall see later, at a very early stage in the 

 development of the embryo of all higher forms we can recognise three 

 layers of cells, namely, an ectoderm, covering the outer surface ; an 

 entoderm, forming the lining of the main part of the gut from 

 pharynx to rectum ; and a mesoderm, which lies between these two 

 layers. Within the mesoderm a cavity, the ccelom, appears, so that 

 it becomes divided into two layers, an outer part helping to form the 

 body wall, and an inner part taking part in the formation of the gut 

 wall, but both, of course, are just parts of one and the same layer. 

 Through the presence of the ccelom, however, it is possible to 

 recognise quite early two parts of the mesoderm that have slightly 

 different fates. The layer lining the ccelom is by its very position 

 an epithelium, and is spoken of as the mesothelium ; while the 

 remaining part between this and either the ectoderm or entoderm 

 is in the embryo a sort of non-specialised padding tissue, and so 

 termed the mesenchyme. The result is then, that we can divide up 

 the tissues of the adult into three main groups, according to the 

 embryonic layer from which they are derived, and one of these, that 

 of the mesoderm, falls into two sub-groups : See over. 



This classification, while not of much assistance in practical 

 work in histology, since it is not based on structural similarity, is 

 of considerable importance in comparative work, and also in a 

 consideration of the nature of the organ and of the diseased or 

 pathological conditions to which it may be subject. 



