4 HISTOLOGICAL TECHNIC 



different elements with dift'erent degrees of intensity. This is known 

 as differential or selective staining. 



The final step in the process is the mounting of the specimen, after 

 which it is ready for microscopic study. 



In this chapter only the more common procedures used in the 

 preparation of tissues for microscopic study are described. At the 

 end of each section are given the technical methods most satisfactory 

 for the demonstration of the tissues described in that section. For 

 other methods the student is referred to special works upon micro- 

 scopic technic. 



Dissociation of Tissue Elements 



Certain of the structural features of such tissues as nerves, muscle, 

 and epithelium, which have but little intercellular substance, or of 

 the looser forms of connective tissue, may be well demonstrated by 

 dissociation. 



This is accomphshed by (i) teasing, or (2) maceration, or both. 



(i) Teasing. — This consists in pulling apart fresh or preserved 

 tissues by means of teasing needles. Instructive specimens of such 

 tissues as muscle and nerve may be obtained in this way. In fact it 

 is feasible to study many tissues in the same manner if tiny pieces 

 are picked into fine shreds on the slide. This method possesses the 

 advantage of exhibiting the tissues in their natural or living condition, 

 whereas most of the methods subsequently described, although 

 having distinct advantages of their own, exhibit the tissues after 

 they|are killed and artificially colored. During the past few years 

 extensive and important observations have been made on cells and 

 tissues while living and actually growing. 



(2) Maceration. — This is the subjecting of a tissue to the action 

 of some chemical which breaks down the substance uniting the tissue 

 elements, thus allowing them either to fall apart or to be more easily 

 dissociated by teasing. The most commonly used macerating fluids 

 are: 



(a) Ranvier's Alcohol (33-per-cent., made by adding 35 c.c. of 

 96-per-cent. alcohol to 65 c.c. of water). — Bits of fresh tissue are 

 placed in this fluid for from twenty-four to forty-eight hours. The 

 cells may then be easily separated by shaking or by teasing. Ran- 

 vier's alcohol is an especially satisfactory macerating fluid for 

 epithelia. ^^ 



