n PERITONEUM- NEURAL CAVITY 27 



tive organs. Leaving these, the peritoneum of the right 

 side approaches that of the left, and the two, coming into 

 contact, form a double vertical sheet, the mesentery (mes\ 

 which extends ventrally towards the enteric canal. On 

 reaching the latter, the two layers diverge again and 

 surround the canal, forming the visceral layer of the perito- 

 neum (v.per). The liver, oviducts, etc., are suspended and 

 covered in the same way. Thus the lining of the body- 

 cavity, the investment of the various organs contained in it, 

 and the folds by which they are suspended, are all parts of 

 one continuous membrane. The space left between the two 

 diverging layers of peritoneum, in the mid-dorsal region, 

 contains lymph, and is known as the sub-vertebral lymph 

 sinus (s. v. ly. s). 



We have already noticed the abdominal and musculo- 

 cutaneous veins. Other veins of greater or less size will be 

 seen everywhere, passing, for instance, to the head and 

 limbs (Fig. 3), and in the mesentery. Running parallel 

 with many of the veins are smaller vessels, many of which 

 have pigment in their walls, and which are of distinctly 

 stouter texture. These are the arteries. They contain little 

 blood in the dead animal, and, owing to the stoutness 

 and elasticity of their walls, do not collapse when empty. 

 Hence they a-re quite easy to see in a frog from which all 

 the blood has been drained, while the thin-walled veins are 

 almost invisible under like circumstances. Finally, there 

 will be seen in many parts of the body, often lying parallel 

 to an artery and a vein, white cords, the nerves. 



The Neural Cavity and its Contents. By turning the 

 frog with its back upwards and cutting through the muscles 

 of the back and the arches of the vertebrae (see Fig. 6), as 

 well as, in front, the roof of the skull, you will see that the 

 backbone contains a distinct cavity, the neural canal, in 



