VERTEBRATED ANIMALS. 281 



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. 



677. Muscular Tissue. Although we are accustomed to speak of this 

 tissue as consisting of ' fibres/ yet the ultimate structure of the ( muscu- 

 lar fibre ' is very different from that of the ' simple fibrous tissues ' al- 

 ready 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. 472), 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 



FIG. 472. FIG 473. 



Fasciculus of stri- Striated Muscular Fibre, separating into fibrillse. 



ated Muscular Fibre, 

 showing at a the trans- 

 verse striae, and at b its 

 junction with the ten- 

 don. 



examined by the Microscope, they are found to be themselves fasciculi,, 

 composed of minuter fibres bound together by delicate filaments of con- 

 nective tissue. By carefully separating these, we may obtain the ulti- 

 mate muscular fibre. This fibre exists under two forms, the striated and 

 the non-striated. The former is chiefly distinguished by the transversely- 

 striated appearance which it presents (Fig. 473), and which is due to an 

 alternation of light and dark spaces along its 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 fibrillse into which the 

 fibre may be broken up. When a fibre of this kind is more closely ex- 

 amined, it is seen to be inclosed within a delicate tubular sheath, which 

 is quite distinct on the one hand from the connective tissue that binds 

 the fibres into fasciculi, and equally distinct from the internal substance 

 of the fibre. This membranous tube, which is termed the sarcolemma, is 

 not perforated by capillary vessels, which therefore lie outside the ulti- 

 mate elements of the muscular substance; whether it is penetrated by the 



