110 THE S^fAIL 



through its thin roof, immediately behind the collar. Extend 

 this incision backwards along each side of the cavity to its 

 jposterior end, cutting along the left side close to the junction 

 •of the mantle and body-wall, and along the right side about 

 ■a quarter of an inch to the left of the prominent ridge that 

 borders this side, midway between this and the right border of 

 the kidney. 



Turn back the roof of the mantle-cavity, and pin it down 

 ■SO as to fully expose the pericardium and kidney. 

 A. The Walls of the Mantle-cavity. 



1. The roof and sides of the cavity are formed by the 



mantle itself, a thin fold of skin which bears on its 

 under surface a very rich network of large blood- 

 vessels, especially abundant in front and along the 

 right side. 



These vessels, in which the blood is aerated, unite 

 to form a large trunk, the pulmonary vein, which 

 takes back the arterialised blood to the heart, run- 

 ning along the left side of the kidney. 



The mantle is greatly thickened in front to form 

 the collar : posteriorly it is thickened by the kidney 

 and pericardium, both of which lie in its substance. 



2. The floor of the cavity, which is formed by the dorsal 



wall of the body, is muscular, and strongly arched 

 from side to side. Contraction of its muscles causes 

 flattening of the floor, and thus enlarges the cavity, 

 causing an inrush of air through the pulmonary 

 aperture. On relaxation of the muscles the floor 

 resumes its previous convex shape, diminishing the 

 size of the cavity and effecting expiration. 



By alternate contraction and relaxation of the 

 floor of the mantle-cavity the air is continually 

 renewed, and the cavity thus becomes a true lung. 



E. The Organs in relation with the Mantle-cavity. 

 1. The pericardial cavity and heart. 



a. The pericardial cavity, which has already been seen 

 from the exterior, ^is oval, thin- walled, and half 



