SUGGESTIONS FOR THE LABORATORY ASSISTANTS 141 



pancreatin have always been found reliable. A small percentage of human 

 beings have no ptyalin in their saliva. 



6. Section on tissues. This work always proves rather difficult, chiefly 

 because students use too large pieces. Verbal instructions as to the method of 

 procedure will probably be more effective than those written in the outline. 

 Shed epidermis of the frog will be found in the water in which frogs have been 

 kept for a short time. To prepare tissues from the intestine, proceed as follows: 

 Pith a frog. Cut out the small intestine, slit it open, and wash it in physio- 

 logical salt solution. For columnar epithelium put it into 5 per cent chloral 

 hydrate for twelve to eighteen hours. For smooth muscle, scrape off the mucosa 

 and place the rest, consisting of the muscular coats, in 5 per cent chloral hydrate 

 for twenty-four to forty-eight hours. Connective tissue is always best obtained 

 from a frog which has been preserved in formalin; and striated muscle is usually 

 better from such a source, unless the fresh muscle is carefully handled. Fresh 

 slices of cartilage are infinitely better than any permanent preparations; they are 

 obtained from the ends of the long bones of the frog with a sharp razor. Fresh 

 blood may be obtained by cutting off the toes of a pithed frog and pressing the 

 bleeding ends of the toes against a slide. It is very necessary that the physio- 

 logical salt solution used for blood be exactly isotonic with the blood, or else the 

 blood cells will become distorted. The myelin sheaths of fresh nerves usually 

 become distorted, producing appearances commonly mistaken for nodes of 

 Ranvier. Students almost always have trouble with the Golgi preparations of 

 nerve cells, often mistaking meaningless deposits of silver on the sections for 

 nerve cells. 



7. Section on detailed anatomy. For the study of the circulatory system, 

 injected frogs are essential. To inject a frog, etherize it, cut off the tip of the 

 ventricle and thrust the cannula of the injection syringe up into the conus 

 arteriosus. All of the arteries and the postcaval vein and its branches are injected 

 in this manner. Veins are best studied in freshly etherized frogs, although they 

 are fairly satisfactory in injected specimens if these are injected shortly before 

 using. We have sometimes let the students work out the veins on freshly 

 etherized frogs and then injected the arteries on the same frogs. This has proved 

 satisfactory. After the frogs are injected a piece of the skull should be removed 

 and the frogs placed in fairly strong formaldehyde, about 8 per cent. This 

 serves not only to harden the injected vessels but also to harden the central 

 nervous system, which can then be studied on the same specimens. The injec- 

 tion mass is made of 800 c.c. glycerin, 1,600 c.c. of water, 3 boxes of cornstarch, 

 50 c.c. of carbolic acid (melted crystals) or thymol, and enough coloring matter 

 of the color desired (Berlin blue, carmine, chrome yellow, etc.) to give a deep 

 brilliant color to the whole. Stir up before using. In the case of the frog it is 

 not necessary to tie the cannula in the conus nor to tie up the conus after with- 

 drawing the cannula, but simply hold the cannula in with the fingers. 



