416 



THE A3IEK1CAN MONTHLY 



[Dec. 



8. Prinui PAL Parts OFTHE Trunk AND Post- Abdomen.- 

 — Split tlic smelt ill two ill H vertical plane, pH'^siiig a lii lie 

 to one side of the middle line. C'lt across the ribs of one 

 side and remove the lesser half entirely from the body. 

 Notice that, the post-abdomen is made up entirely of 

 large masses of bluish grey muscnlar tissue, and that the 

 same is true of the dorsal portion of the trunk region; 

 but that in the trunk below the back-bone the muscular 

 tissue is pusiied aside, forming a thin layer in the wall 

 of the body-cavity {coelo/u),nnd that the space thus gained 

 is occupied by the viscera (organs of digestion, etc. The 

 viscera will he studied later). 



Locate the back-bone spinal-column, extending the 

 length of tiie body from the brain case in front to the base 

 of the caudal tin. Examine it and recognize the situa- 

 tion of the successive piecetj, centra, of which it is made; 

 each centrum bears on tlie dorsal side a neural spine 

 which runs u[)W;ird and backward, the course of the neural 

 spine is often indicated by blood vessels which run beside 

 it. In the po.st-al)dominal region a siinilar series of 

 spines run ventrally, these are called haemal spines be- 

 cause the dorsal aorta is related to them. In the trunk 

 region there are no haemal spines; but the ribs on each 

 side in pairs articulate with the centr.i; the ribs pass in- 

 sensibly into haemal spines. Follow the body-cavity for- 

 wards and note that it runs between and beneath the 

 gilL-, where yon will see the deep red heart. It extends 

 posteriorly to the cloaca. It is everywhere lined with 

 a delicate silvery lining layer, the peritoneum. Dissect 

 away some of the peritoneum and see that the wall is 

 composed of muscular tissue, and that this is composed 

 of short fibres which run from one rib to the next (><eea 

 better in a specimen which has been boiled, vid. par. 12). 

 Note that there is no breast l)one for articulation with 

 the ribs at their distal extremitifs; also that the body 

 cavity is not divided by a diaidiragm into thorax and 



