114 HOW TO WORK 



208. other Metallic Salts. Tissues may also be impregnated 

 with solutions of other metallic salts. Acetate of lead has often 

 been employed. The tissue may be soaked for some time in a weak 

 solution, or a weak solution with a little glycerine may be injected. 

 When the tissues are well saturated, thin sections may be made, 

 and, after having been slightly washed, they maybe placed in a dilute 

 solution of glycerine, through which sulphuretted hydrogen may be 

 passed. Living plants will take up solutions of various metallic salts, 

 which may then be precipitated in the textures or in the channels by 

 the appropriate reagents. 



209. modification of the foreg-oin^ Plans. The observer will 

 perceive that these processes are capable of being modified in many 

 ways. Every one engaged in a special investigation, will naturally 

 try various modes of preparation. Having decided upon one which 

 seems to offer considerable advantages, he will try various modifica- 

 tions until he meets with success. I have not attempted to give the 

 minute recommendations of various observers who have employed 

 these processes ; but merely indicate the general outline of the 

 methods. A few experiments will teach the observer more than the 

 most minute directions, and, however carefully directions may be 

 given, it is seldom that anyone succeeds the first time he endeavours 

 to follow them out. Those who desire to do real work in this depart- 

 ment, must be patient, and must work on steadily, until they meet 

 with success. 



OF DEMONSTRATING THE DIFFERENT STRUCTURES OF THE 



HIGHER ANIMALS, 



General Observations, 



21. On structure. The various tissues and organs of animals 

 and plants for the most part are compound, and made up of 

 several distinct elementary structures. For example, the smallest 

 portion of flesh or muscular tissue, which can be removed with a 

 knife or pair of scissars, is composed of several distinct structures. 

 In the first place must be noticed the proper substance peculiar to 

 muscular tissue, in which the characteristic contractile power resides. 

 Secondly, at least in most cases, we find a tube composed of perfectly 

 clear, transparent, almost structureless membrane, in which this con- 

 tractile substance, or sarcous matter, is contained. Thirdly, there 

 exists a certain quantity of areolar or connective tissue, which con- 

 tinuous in structure with the sarcolemma, connects together the.-L- 

 elementary fibres ; and not unfrequently associated with this is a 

 little fatty or adipose tissue. Fourthly, are vessels lying between the 

 elementary fibres just described, in which the blood circulates, for 



