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 enlarged 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 J. 



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 



