xiii.] THE FROG. 255 



e. Examine longitudinal sections of the femur or 

 humerus : the Haversian canals are seen to be 

 channels running for the most part in the long 

 axis of the bone, but communicating with one 

 another frequently by cross branches. The 

 lacuna^ &c. appear much as in the transverse 

 section. 



d. Connective tissue. 



i . Of this there are two main varieties, viz. 



a. White fibrous tissue. This occurs nearly pure in 

 tendons, but is widely distributed throughout 

 the body, mixed with other tissues. Tease out 

 a bit of fresh tendon in water: examine with a 

 high power. 



a. It is chiefly made up of very fine wavy fibres 

 which run in bundles parallel to one another; 

 they have an ill-defined outline and do not 

 branch. 



ft. Treat with dilute acetic acid. Most of the 

 fibres disappear, but a few well-defined curled 

 fibres (yellow elastic fibres ; see b) remain. Be- 

 sides these some elongated granular proto- 

 plasmic masses are brought into view (con- 

 nective-tissue corpuscles). 



b. Yellow elastic tissue. This does not occur in large 

 collections, in the pure form, in the frog; although 

 mixed with white fibrous and other tissues, it is 

 very widely distributed. 



a. Tease out in acetic acid some of the bands of 

 tissue beneath the frog's skin; examine with 

 a high power. Numerous fine well-defined 



