1646 A MANUAL OF ANATOMY 



the free lateral portion of the dorsal margin of the septum trans- 

 versum. In this manner the pleural compartments of the ccelom 

 are shut off from the peritoneal compartment, and the diaphragm 

 is completed dorsally. At an earlier date the pleural cavities are 

 shut off from the pericardial cavity by the pleuro-pericardial mem- 

 brane. The four divisions of the originally single coelom or body- 

 cavity now constitute four serous sacs, quite independent of each 

 other — namely, pericardial, pleural, right and left, and peritoneal. 



From what has been stated it will be evident that the pleural 

 sacs or cavities are originally thoracic prolongations of the peritoneal 

 cavity, from which they are shut off by the pleuro-peritoneal mem- 

 branes. At the caudal end of the peritoneal cavity there are two 

 somewhat similar scrotal prolongations, called the processus vaginales, 

 one on either side. These prolongations are, as a general rule, shut 

 off from the peritoneal cavity by the closure of their upper ends. 

 The lower part of each process forms the tunica vaginalis. 



The corresponding prolongations in the female are known as the 

 canals of Nuck. 



The diaphragm, as stated, is developed in two parts, ventral and 

 dorsal. The ventral portion is formed from the middle layer of the 

 septum transversum, whilst the dorsal portion is developed from the 

 two pleuro-peritoneal membranes. In very rare cases the dorsal 

 portion of the diaphragm may fail to unite with the ventral portion 

 on one or both sides, due to an arrest in the development of one 

 or both pleuro-peritoneal membranes, and this condition is spoken 

 of as an arrest in the development of the diaphragm. Under such cir- 

 cumstances a communication exists between the peritoneal and 

 thoracic cavities. If a part of an abdominal viscus should protrude 

 into the thoracic cavity through such an aperture, the condition is 

 known as a congenital diaphragmatic hernia. 



Musculature of the Diaphragm. — ^As stated, the septum trans- 

 versum is, at first, on a level with the third and fourth cervical 

 somites. The muscular tissue of the diaphragm is, in part, derived 

 from the myotomes of these segments, especially the fourth seg- 

 ment, and, it may be, the fifth also, which explains why the muscle 

 is supplied by the phrenic nerve, the two chief roots of that nerve 

 being derived from the anterior primary divisions of the third and 

 fourth cervical spinal nerves. 



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