AN INTRODUCTION TO ZOOLOGY 



The muscles in the frog are fibrous in structure and of two 

 kinds. The first are those forming the muscles of the body wall 

 and limbs, and their fibres, grouped in bundles, appear transversely 

 striped when examined under the microscope. They are conse- 

 quently termed striate or, because they can be made to contract 

 at will, voluntary muscles. The second kind is found in the walls of 

 the alimentary canal, the bladder and blood-vessels, and does not 

 appear striped. They are called non-striate muscles, and, as they 

 are not under the control of the will, involuntary. In general we 

 find that striate muscles are directly or indirectly 

 connected with the skeleton, whereas the non-striate 

 are related to the internal organs, and hence the 

 two are sometimes distinguished as skeletal and 

 visceral respectively. 



Involuntary muscles consists of a number of 

 long spindle-shaped cells about I mm. in length, 

 with pointed ends, that may in rare instances be 

 bifurcated. The cell body is enclosed by a definite 

 membrane, and within it is situated a characteristic 

 elongated nucleus. The cell as a whole is highly 

 refractive, but distinctly granular in the neighbour- 

 hood of the nucleus, and may appear very faintly 

 striated in the longitudinal direction, owing to the 

 presence in it of a number of extremely fine fibrils. 

 The cells are situated in layers closely bound to- 

 gether, so that their individual outlines cannot be 

 made out save in preparations from material that 

 has been allowed to stand in certain solutions * and 

 then teased, i.e. torn up with fine needles. They lie 

 parallel with one another, and are bound together 

 by a small amount of intercellular substance. Such 

 muscles do not appear to be capable of quick move- 

 ment in unison, but are adapted for slower ones, 

 such as are required in the blood-vessels and bladder, or the 

 rhythmic contractions, peristalsis, that pass along the alimentary 

 canal during digestion. 



' Many of the skeletal muscles, such as the biceps, the large muscle 

 that draws up the fore-arm, and the gastrocnemius, that forms the 

 calf of the leg, have a very characteristic shape. They are spindle- 

 shaped, tapering off at each end and swelling out in the middle to 

 form the belly. The ends are continued on as the connective 

 tissue ligaments, and these in turn are joined to the skeleton. When 



* Such a process is termed maceration. The fluid may be very weak spirit 

 or a weak solution of Potassium bichromate. 



FIG. ii. Two 

 non - striate 

 muscle cells 

 from wall of 

 i n testine 

 showing 

 nuclei and 

 longitudinal 

 striations. 



