108 ATLAS OF BIOLOGY. 



C. Whole animals, or dissections, are best preserved in alcohol. The ordinary methylated spirit of com- 

 merce, diluted with ^rd its bulk of water, answers admirably ; delicate structures, such as the brain, must be 

 placed at once into strong methylated spirit, and in all cases the preservative medium should be replaced after 

 the first twenty-four hours. With the above exceptions, it is not desirable to submit any preparation at once 

 to the action of strong methylated spirit, as such a course is apt to result in a too rapid dehydration of the 

 superficial parts, thus preventing that complete permeation necessary for successful preservation. 



Where very careful preservation without shrinking is desired, it is well to use in succession, 50 p.c. for 

 twenty-four hours, 75 p.c. for the next forty-eight, methylated spirit ad infinitum, being careful to prevent the 

 access of air. 



D. For ordinary coarse injection of the blood vessels, a mixture of French blue, in the proportions of a 

 teaspoonful to a tumbler of water, cannot be surpassed. It is cheap, sufficiently finely divided to enter 

 all but the capillary vessels in a small animal, and it can be used with little trouble. Moreover, the fact 

 that it is insoluble in water renders it the more valuable, as injection can be carried on piecemeal as circum- 

 stances may require. 



In dealing with the animals adopted for this tcok, a i-mall canula, provided with a tight-fitting india- 

 rubber ball or nipple, as figured below, is all-sufficient as a syringe. 



For histological purposes, a gelatine injection is best suited, made as follows : Allow a given quantity of 

 Nelson's gelatine to stand fcr twenty-four hours in twice its bulk of water, toil and stir well, adding either 

 French blue or vermilion to colour as required. This fluid should be injected at a temperature such as the 

 hand can comfortably bear. 



In preparing a Frog for injection with gelatine, an incision should first be made along the whole ventral 

 integument (care being taken to injure nothing else), and the two halves reflected. Eaising the xiphoid process 

 with a pair of forceps, next insert the point of the scissors under this, and carefully remove the whole ventral 

 portion of the shoulder-girdle. By this means the heart will be exposed without injury, the pericardium 

 should now be laid open, and an incision made into the apex of the ventricle, to allow the escape of as much 

 blood as possible. When the animal is sufficiently well bled, a canula* should be tied into the heart, and 

 connected, by means of an inch or two of india-rubber tubing, with the syringe, injection being performed 

 under a gentle, steady pressure. 



When the syringe is removed, the tube by which it was connected to the canula should be plugged with 

 a piece of glass rod, and the whole animal placed in water for two or three hours, that the injection may set. 

 After the above treatment, the parts to be preserved should be dealt with as recommended for the uninjected 

 tissues. 



E. The term branch as applied to blood-vessels is often very vaguely employed. Defining an artery as a 

 vessel carrying blood from the heart, and a vein as one conveying it to the heart, it will be well to restrict 

 the term branch to those vessels formed by the breaking up of an artery into smaller trunks, and that of 

 factor to those uniting to form larger veins ; arteries and branches being efferent, factors and veins being 

 afferent, as related to the heart. 



F. By a wet preparation is meant one that has never been allowed to dry. 



The skeleton of the Frog may be readily prepared by allowing the body to lie in water for a few days, 

 after having first removed the skin, viscera, and as much of the flesh as possible. Maceration having 

 gone as far as is desirable, the whole should be put under running water for a day, before it is allowed to 

 dry. 



* This mst be made to the required size by drawing out a piece of glass tubing, either in a flame or by means of the blow- 

 lould be shghtly constricted near the apex, to allow of its being firmly tied into the heart. 



