1048 VEKTEBRATED ANIMALS 



are well displayed in injected preparations. For such a full and 

 complete investigation of the structure of these organs as the 

 anatomist and physiologist require, various methods must be put 

 in practice which this is not the place to detail. It is perfectly 

 easy to demonstrate the cellular nature of the substance of the 

 liver by simply scraping a portion of its cut surface, since a number 

 of its cells will then be detached. The general arrangement of the 

 cells in the lobules may be displayed by means of sections thin 

 enough to be transparent ; whilst the arrangement of the blood- 

 vessels can only be shown by means of injections. Fragments of 

 the tubules of the kidney, sometimes having the Malpighian cap- 

 sules in connection with them, may also be detached by scraping its 

 cut surface ; but the true relations of these parts can only be shown 

 by thin transparent sections, and by injections of the blood-vessels 

 and tubuli. The simple follicles contained in the walls of the 

 stomach are brought into view by vertical sections ; but' they may 

 be still better examined by leaving small portions of the lining 

 membrane for a few days in dilute nitric acid (one part to four of 

 water), whereby the fibrous tissue will be so softened that the 

 clusters of glandular epithelium lining the follicles (which are but 

 very little altered) will be readily separated. 



Muscular Tissue. Although we are accustomed to speak of this 

 tissue as consisting of 'fibres,' yet the ultimate structure of the 

 ' muscular fibre ' is very different from that of the ' simple fibrous 

 tissues' already described. When we examine an ordinary 

 muscle (or piece of ; flesh ') with the naked eye, we observe that it 

 is made up of a number of fasciculi or bundles 

 of fibres (fig. 784), which are arranged side by 

 side with great regularity, in the direction in 

 which the muscle is to act, and are united by 

 connective tissue. These fasciculi may be 

 separated into smaller parts, which appear like 

 simple fibres ; but when these are examined by 

 the microscope, they are found to be themselves 

 fasciculi, composed of minuter fibres bound 

 together by delicate ' filaments of connective 

 tissue. By carefully separating these we may 

 obtain the ultimate muscular fibre. This fibre 

 exists under two forms, the striated and the 



FIG. 784. Fasciculus non -striated. The former is chiefly distinguished 



of striated muscular by the transversely striated appearance which 



Snsvtr^sfri^a^d ^ P r6Sents (%'. 785 )> and which is due to an 



at 6 its junction' with alternation of light and dark spaces along its 



the tendon. whole extent; the breadth and distance of 



these striae vary, however, in different fibres, 



and even in different parts of the same fibre, according to their 



state of contraction or relaxation. Longitudinal striae are also 



frequently visible, which are due to a partial separation between 



the component fibrillae into which the fibre may be broken up. 



When a fibre of this kind is more closely examined, it is seen to be 



inclosed within a delicate tubular sheath, which is quite distinct on 



