14 PRACTICAL WORK CHAP. 



Rules to be Observed in Dissection. Many of the parts and 

 organs of animals are bound together by means of a substance known as 

 "connective-tissue," and the main object of dissection is to tear away 

 and remove this substance so as to separate the parts from one 

 another. 



The subject should be firmly fixed down in the dissecting-dish or on 

 the dissecting-board by means of pins, inserted obliquely, so that they 

 do not interfere with the dissection. The dissecting-dish must always 

 be used for finer dissections, which should be done under water ; only 

 just enough water being put into the dish to cover the dissection, which 

 should be washed under the tap from time to time. 



When dissecting a part keep it on the stretch, and avoid fingering it or 

 damaging it with the forceps. 



Never remove anything until you know what you are removing. 

 Dissect along and not across, such structures as blood-vessels and nerves. 



See that your instruments are kept clean and sharp, and never use the 

 smaller scissors and scalpels for coarse work. 



Drawing. You should make a point of drawing as many of your 

 preparations, as well as of the living animals, as possible : an accurate 

 sketch, taken from nature, no matter how rough, is of more value in 

 teaching observation and in impressing the facts on your memory than 

 the examination and copying of more perfect drawings made by others. 

 Anyone can soon learn to make sketches of this kind, even without 

 having any previous knowledge of drawing. 



Each sketch should be made to scale, and small objects should be en- 

 larged several times ; it is much easier to insert details in a large drawing 

 than in a small one. Mark the scale against each drawing e.g., x 2, 

 x*. 



Using a rule and compasses, first sketch in an outline of the principal 

 parts with a hard pencil ; if your object is bilaterally symmetrical, draw 

 a faint line down the middle of the paper, and then sketch in one side 

 first. When you have sketched in all the outlines correctly, go over 

 them again with a softer pencil, so as to make them clear and distinct. 

 Do not attempt any shading unless you have some knowledge of 

 drawing. 



Then tint the various parts in different colours, using very light tints 

 except for such structures as vessels and nerves. You should keep to 

 the same colours for the corresponding organs or tissues in all the 

 animals you examine : thus you might in all cases colour the alimentary 



