240 PRACTICAL ANATOMY. 



This is as follows : (i) Organs may simply be bcliind the peritoneum, as arte- 

 ries and veins, in which case we say they are covered by this membrane. (Fig. 

 1 68.) They have not grown sufficiently in thickness to produce any appreciable 

 displacement of the peritoneum. Figure 168 shows the aorta behind the peri- 

 toneum, but producing no bulging forward of the peritoneum. (2) Organs may 

 produce appreciable bulging forward, as in figure 167, when we say they are 

 partially invested. Now examine the kidney and you will find its relation to the 

 peritoneum represented in figure 167 in the two organs on the two sides. 



7. What determines the degree of peritoneal investment .' 



The height to which any organ grows determines this. The peritoneum 

 pushed ahead of the organ is called the mesentery of the organ and the peritoneal 

 investment. Mesenteries may be long or short. The height to which an organ 



PARIETAL LAYER OF PERITONEUM 



A MESENTERY 



The aorta 



FIG. 169. 

 In this figure an organ has grown to a considerable height and a long mesentery is the result. 



PARIETAL PERITONEUM 

 VISCERAL PERITONEUM 



The aorta 

 FIG. 170. 



In this figure, a is an organ completely invested by peritoneum. Its mesentery grows high and falls on b, 

 an organ partially invested by peritoneum. At the contact point of a and b loss of epithelium occurs. 

 Parts of organs, or entire organs, which once possessed peritoneal investment, but subsequently lost 

 the same in this manner, are said to be retro-peritoneal, or to have no peritoneal investment. An 

 example is to be seen in the case of the right kidney, where the duodenum and colon bear the above 

 relation (Fig. 170) to the anterior surface of this organ. 



grows determines, then, both the degree of investment and the length of the 

 mesentery. 



8. How are the abdominal muscles classified? 



As vertical and horizontal. The pyramidalis and rectus abdominis are ver- 

 tical ; the external and internal oblique and the transversalis are horizontal. 



9. Name the abdominal weak points in the ivalls of the abdominal cavity and 

 give their practical importance in surgery. 



(i) The inguinal canal, limited internally by the internal abdominal ring, and 

 externally by the external abdominal ring. This canal lodges the spermatic cord 

 of the male and the round uterine ligament of the female. It is the succession 

 of weak points that formed the line of least resistance to the descending testicle. 



