MUSCLE 157 



IV. Glands of Frog's Skin. Sections of the frog's skin 

 are admirably suitable for histological purposes, as they show 

 a collection of typical tissues. 



Prepare sections thus: Take a fresh-caught summer frog ; 

 chloroform it ; cut off a small piece from tlie skin of the back ; 

 fix with corrosive acetic acid ; stain with hcematoxylin, or stain 

 the sections on the slide. 



The Epidermis is stratified, pitted into two sorts of 

 glands. 



1. Granule glands. Very large, probably poisonous ; sacs 



without an opening to the duct. They disappear 

 in captivity. 



2. Slime glands. Flask -shaped sacs that produce the 



mucus with which the body is covered and safe- 

 guarded from the germination of parasites. 



The pigment-cells, muscle-cells, and blood-cells are excel- 

 lently shown. 



C. Muscle. 



In muscular tissue the component ceils are much elongated 

 and, in the higher forms, very highly specialised. Muscular 

 tissue is of two kinds : (1) striated, or voluntary, of which all 

 muscles that are under the control of the will consist : and 

 (2) non-striated or involuntary, forming those muscles over 

 whose contractions the will has no direct control. The 

 muscular tissue of the heart, which though involuntary is 

 striated, forms the chief exception to this rule. 



I. Striated, or Voluntary Muscle. 



a. Crab's muscle. Shred in glycerine a small piece of 

 crab's muscle that has been hardened in alcohol; 

 cover, and examine with both low and high powers : 

 note : 



i. The elongated fibres of which the muscle consists. 

 Each fibre is a single cell, and is enclosed in a 



