14 THE AMERICAN MONTHLY [Jan, 



push tKem aside and pin down, so as to fully open the 

 cavity. Note that there is no diaphragm subdividing the 

 body cavity into thorax and abdomen; note also the 

 mesentery, its thin delicate texture, and its continuation 

 out onto the body wall where it passes insensibly into the 

 peritoneal lining. In dissecting the contents of the body 

 cavity do not cut any of the organs away, till after you 

 have located and examined the relations of them allj 

 merely dissect them apart, and push them aside to see 

 underlying ones. After all the points have been noted 

 you can t-hen cut out such as are necessary. 



5. The Heart is located in the neck very close be- 

 hind the; head; it is next the ventral body wall and in 

 front of the level of the anterior limbs. In the construc- 

 tion of its interior it is also intermediate between the 

 single circulation type as in the smelt and the complete 

 double circulation of the bird or mammal. Keroove the 

 pericardiiim\ this will enable you to see the different parts 

 of the heart and some of its large communicating vessels. 

 There is in front a pair of aortic arches which lead out 

 from a distinct biilbus arteriosus. Posterior to the bulbus 

 is the single ventricle^ it lies on the right side and some- 

 what veutrally to the auricles. There are two auriclesj 

 the veins from the body at large empty into the right 

 auricle; the blood from the lungs empties into the left 

 auricle. If the specimen is in a suitable condition cut 

 the chambers open and using a probe carefully trace their 

 connections both with each other and with the large 

 communicating blood vessels. Both auricles open into the 

 single ventricle (but in such a manner as to send the best 

 aerated blood to the head, and the poorest to the posterior 

 parts of the body). 



[6. The Vascular System, — can only be adequatHy 

 dissected upon an injected specimen, but an outline 

 description is included here for convenience and a good 



