I.] I.] THE FROG. 133 



b. Obtain a similar but thin piece from the intestine 

 of a recently-killed frog. Treat with silver 

 nitrate as directed at § 2. c. 



a. The cementing substance; obvious as a series 

 of dark lines which cross each other at long 

 intervals. 



/?. Look for fragments showing two sets of lines 

 which cross at right angles (i.e. cell-xontours of 

 longitudinal and circular layers). 



8. Striped muscle. 



a. Preserve the middle third of the belly of a limb- 

 muscle (say the gastrocnemius) in spirit ; stain with 

 borax-carmine and cut into transverse sections. 

 Mount in Canada balsam and examine under a 

 low power. 



u. The muscle ; composed of a great number of 

 large fibres, each oval, or angulated and irre- 

 gular as the result of mutual compression. Note 

 their transverse diameter, variable in proportion 

 as the section passes through their middle 

 or extremities. 



/J. The perimysium {fascia) ; a deeply staining 

 sheath for the whole muscle, carrying blood- 

 vessels whose cut ends will be seen (cf. peri- 

 osteum, § 5.). It will be found to dip into the 

 interior of the muscle as a delicate black pig- 

 mented membrane, subdividing it into a number 

 of muscle bundles or fasciculi. 



y. The muscular fibres. Examine the thinnest parts 

 of your sections with a high power ; each fibre 

 consists for the most part of a granular pro- 



