70 VERTEBRATE ANATOMY 



Exercise XIV 



we will examine such sections in greater detail 

 later, so don't spend too much time with them 

 now. 



The animals will be given to you alive but 

 anesthetized. One of the problems that biologists 

 face, and that you are facing now, is how to 

 examine what goes on inside animals without 

 causing them pain. "Anesthesia" literally means 

 lack of feeling or sensation. We try to achieve 

 this either by rendering the higher centers of the 

 brain functionless with the use of a narcotic 

 ("anesthetic") or by destroying those centers. 

 Whatever one does in this regard should be done 

 skillfully and quickly. If you pith your own frog, 

 know exactly what you are going to do, and be 

 ready to do it before you make the first move. 

 All the biologists we know take a lot of trouble 

 with such procedures. Speed and skill are of the 

 essence. If an animal needs to be killed in the 

 course of a laboratory procedure, one takes 

 similar precautions, trying whenever possible to 

 kill in one stroke an animal that has been handled 

 gently up to that point. 



The particular point of the procedures we have 

 used here with the rat and frog is to abolish pain, 

 yet permit you to examine the organs in a func- 

 tional state. Take full advantage of this op- 

 portunity, and be prepared to begin as soon as 

 the animal is ready. Work as fast as you can, 

 consistent with care and thoroughness. 



The rats will be given an overdose of bar- 

 biturate by the instructor at the start of the 

 period. Watch the anesthesia take hold. This in 

 itself is instructive. The barbiturates inhibit the 

 higher centers of the brain first — the cortical 

 centers — lessening the animal's coordination. 

 The first effect is a staggering gait. (This is what 

 ethyl alcohol does to us.) Gradually the animal 

 becomes immobile, though its reflexes still re- 

 spond to external stimuli. The reflexes are con- 

 trolled through lower centers of the brain or 

 through the spinal cord, both of which are more 

 resistant to narcosis. Eventually the reflexes also 

 cease to respond, yet the animal continues to 

 breathe. The respiratory center in the brain 

 stem is extremely resistant to narcosis. Why do 

 you think this is so? 



At this point, when the animal no longer 

 responds to stimuli, but is still breathing, begin 

 the dissection. 



The frogs are "anesthetized" by destroying 

 (pithing) their brains. One of the instructors will 

 demonstrate this procedure, and will pith your 

 own frog if you wish. 



Pithing is done by quickly inserting a dissecting 

 needle, directed forward into the skull, at the 

 point where it joins the backbone, meanwhile 

 moving the point of the needle from side to side 

 as far as it will go, to cut as many as possible of 

 the nervous connections. If you hold a frog 

 gently but securely, and bend its head a little 

 downward, you can find a little depression at the 

 back of the head that marks its joint with the 

 backbone. This is where the needle should be 

 inserted, with one swift motion that gets inside 

 the brain cavity, stirring as it advances. 



THE RAT 



First look carefully at the external appearance 

 of your animal, and compare it with the frog. 

 Note diff'erences in texture of the skin. Where 

 does the head end ? What do you see in the way 

 of ears? How many digits are on the feet? Feel 

 such major body landmarks as the rib cage, back- 

 bone, and the connection of the backbone with 

 the skull. Where is the heartbeat strongest? 

 Note the breathing movements. 



Lay the animal on its back, and slit open the 

 skin from the jaw to the genital openings. Be 

 careful not to cut through the underlying tissues. 

 Separate the skin laterally, using your fingers 

 or the blunt end of the scalpel. (If you have 

 difficulty here, the instructor will demonstrate.) 



Note: The popular image of a biologist has 

 him dissecting with a scalpel. The truth is that 

 the cutting edge of the scalpel is used relatively 

 little. Most cutting is done with scissors, and 

 most dissecting is done by prying, pushing, and 

 lifting things apart rather than cutting. You will 

 probably find the spade-shaped, blunt end of the 

 scalpel more generally useful than the blade. 

 When you really want to slice, as, for example, 



