THE TURTLE. 201 



all around the border, and carefully dissect the plastron 

 free from all its attachments, and entirely remove it. 

 See the points where it was connected by ligaments with 

 the pectoral and pelvic arches. Muscles already severed 

 from their origin on the plastron cover the bones of these 

 arches. Dissect these muscles carefully away, noting 

 that they are opposed in pairs on each half of each arch. 

 Find nerves and blood vessels entering them internally. 

 Avoid cutting any large branches of the blood vessels. 



The peritoneum, covering the posterior part of the body 

 cavity, should still be intact, and on its inner surface a 

 pair of abdominal veins should be plainly seen. 



The heart should be seen still pulsating in its thin 

 pericardium, in the anterior part of the body. 



The abundant transparent liquid which fills the spaces 

 within pericardium and peritoneum is lymph. 



Placing a thumb inside each half of the pectoral arch, 

 press outward until its ligamentous attachment with the 

 inside of the carapace is broken. Then draw the fore 

 legs forward and outward, to separate the halves of the 

 pectoral arch, and fasten them so. 



I. The Heart. Open the pericardium, and fully dis- 

 close the heart and its connections. Observe : 



1. A single large posterior ventricle. 



2. A pair of smaller auricles. 



3. White, elastic arteries arising from the ventricle. 



4. Thinner-walled, darker-colored veins entering the 

 auricles, and expanded, just before they enter, to form a 

 contractile venous sinus. 



Note the order of contraction, first the venous sinus, 

 then the auricle, then the ventricle. This wave of con- 

 traction pushes the blood forward, and indicates the direc- 

 tion in which it is flowing through these parts. If the 

 blood vessels be now injected, they will be much more 

 easily traced later. 



