THE FROG 17 



underneath which lies the heart. The two hindermost portions of 

 the pectoralis muscle appear to form the body wall, but it will be 

 seen, if they are carefully removed with the scissors, that the true 

 body wall is beneath. If a small portion of the body wall be 

 removed, exposing the internal or body cavity, placed on a glass 

 slide and examined with a lens against the light, it will be seen to 

 consist of two layers of muscles. The outer of these is composed of 

 a thin layer of fibres, the obliquus externus, the grain of which runs 

 obliquely upwards and outwards. The external oblique has under- 

 neath it an obliquus interims, whose grain runs outwards and down- 

 wards, and so the two- together form the wall of the body. 



An incision may now be made in the body wall on one side 

 close to the linea alba from the hinder end forwards. In the region 

 of the middle division of the pectoralis muscle a bone will be en- 

 countered. This must be cut through with a strong pair of scissors, 

 care being taken not to go too deeply and damage the underlying 

 structures. A short distance in front of this is another bone that 

 may be cut through in the same way. These two bones form part 

 of a bony framework, the pectoral girdle, that supports the fore- 

 limb. A more lateral cut may now be made, great care being taken 

 not to injure any of the large blood-vessels connected with the arm, 

 and so a piece of the ventral body wall will Ue removed. A similar 

 operation can be performed on the opposite side, thus leaving only 

 a narrow strip of tissue in the middle line. On turning aside this 

 strip in the region of the front end of the linea alba you will see that 

 the anterior abdominal vein leaves it and goes down into the organs 

 of the body. By cutting across the strip just in front of this point 

 the anterior part of it may be removed.* 



By thus removing strips of the body wall a large hollow, the body 

 cavity, coelom or pleuro-peritoneal cavity, will have been opened up. 

 In it are situated the various internal organs or, as they are com- 

 prehensively termed, the viscera. In ourselves it is completely 

 divided into two cavities, a pleural and peritoneal, by a transverse 

 sheet of muscles, the diaphragm. The purple-coloured heart will be 

 seen situated close to the sternum in a fairly tight fitting membranous 

 bag, the pericardium. Within this bag is a hollow, the pericardial 

 cavity, actually a part of the general ccelom that has been cut off. 

 The heart is composed of three separate chambers, a thick-walled 

 posterior pointed chamber, the ventricle, and two more anterior thin- 

 walled auricles. From the top right-hand side of the ventricle a 



* To obtain more freedom in the examination of the various structures, the 

 linea alba with its accompanying vein and small strip of body wall may be 

 tied, i.e. ligatured, in two places fairly close together and then cut between. 

 By this means loss of blood is prevented. 



C 



