ANIMAL BIOLOGY. [Part I. 



With the point of a scalpel a small incision may now be made 

 in the muscular body-walls just in front of the hip-girdle, and 

 sufficiently to the left (the frog's left) of the middle line to avoid 

 injuring the median vein. A small hole will thus be made into 

 the body-cavity or cozlom, which contains the chief viscera. By 

 inserting the small forceps into this hole the body-walls may be 

 raised, and an incision may be carried forward just to the left of 

 the median vein as far as the bony bar of the shoulder-girdle. 

 A similar incision may be made to the right of the median vein, 

 and with a little care it may be dissected away from the strip of 

 the body- wall to which it is attached. On raising the sternum 

 the heart will be seen lying beneath it in a membranous bag. 

 Avoiding injury to this organ, the left incision may now be 

 carried forward by dividing with strong scissors the bony bars 

 of the shoulder-girdle to the left of the sternum. Those to the 

 right may be similarly divided, and the sternum carefully removed. 

 By the removal of the ventral wall of its membranous bag the 

 heart may be displayed. After trimming away the sides of the 

 body-walls the dissection will now resemble that given in outline 

 in Fig. 7. Fig. 8 represents a transverse section of the body of 

 a frog which had lain in a solution of 1 per cent, chromic acid 

 for three or four days till the bones were softened. The student 

 should make other such sections through the eyes, through the 

 tympanic membrane, and through the shoulder-girdle. 



We are now in a position to take a preliminary view of the 

 following organs, or systems of organs : 



1. The Alimentary System. 



2. The Eespiratory System. 



3. The Heart and Circulatory System. 



4. The Urino-genital System. 



5. The Nervous System. 



6. The Skeletal System. 



7. The Muscular System. 



8. The Integumentary System. 



1. The Alimentary System. The alimentary canal is a tube 

 running right through the body from the mouth to the vent, and 



