THE TISSUES. 31 



nected directly, but by means of a separate process with the 

 fibre, which has at the same time itself also become independent, 

 we can see how the more differentiated stage has been brought 

 about. Nerves and muscles seem from this point of view to be 

 the products of the separation of one and the same layer of tissue, 

 which tissue we shall learn later on to know as the " ectoderm." 

 And at the same time a physiological postulate is thus satisfied : 

 for clearly it is impossible to imagine that nerve or muscle once 

 came into existence with their elements totally distinct from one 

 another, and that the connection between them, on which their 

 functions depend, was the result of a later union. 



Muscular Tissue. 



§ 26. 



The morphological elements of the muscular tissue are, so far 

 as their more special characters are concerned, divisible into ' 

 two groups. One consists of cells simple in form, the other of 

 fibres derived from cell-aggregates, or from syncytia; the latter 

 is indicated by the presence of numerous cell-nuclei. In either case 

 the amount of protoplasm, which retains its indifferent character, 

 is slight, and subordinate in importance as far as the function of 

 the form-elements in question is concerned. Further differentiation 

 of the contractile substance may in either case lead to the higher 

 development of the fibre. 



1) The so-called smooth muscular fibres, or contractile 

 fibre-cells, constitute the first form. They are spindle-shaped 

 cells, which are often greatly elongated, and then are band-like in 

 form ; in these cells either none of the indifferent protoplasm at all 

 persists, or what does is to be found in the long axis, or at the 

 periphery of the cell only. In all cases such remaining protoplasm 

 surrounds the nucleus. The contractile substance is homogeneous 

 and limited externally by a membrane, which is often difficult to 

 demonstrate. The reaction of these muscle-fibres to nerve stimu- 

 lation is slow. 



Owing to differentiation of the contractile substance into singly 

 and doubly refractive particles, the fibres gain the appearance of 

 transverse striation ; such is the origin of that variety of the 

 tissue, which is known as transversely-striated muscular fibre. 

 There are various intermediate forms between this kind of striated 

 tissue, which consists of fibres derived each from a simple cell, and 

 the other more homogeneous kind of fibrous muscular tissue. 



2) The elementary parts of the other form of muscular tissue 

 are formed by cell-aggregates (syncytia). They generally arise, as 

 it seems, from the growth of one cell, the nucleus of which multiplies, 

 so that they may be regarded as arising from the continuous but 

 imperfect division of one cell. Their contractile substance either 

 has a cylindrical shape, is limited externally by a homogeneous 



