ii PERITONEUM NEURAL CAVITY 27 



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 

 peritoneum (v. per). The liver, oviducts, etc., are sus- 

 pended 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 sus- 

 pended, 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 are 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 cir- 

 cumstances. 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 as 

 well as, in front, the roof of the skull (see Fig. 6), you 

 will see that the backbone contains a distinct cavity, the 

 neural canal, in which lies a white rod, made of the same 

 soft kind of substance as the nerves, and called the spinal 



