THE DOGFISH 19 



centrum, circular in outline, and extending dorsally from it 

 a neural arch, enclosing the spinal cord; and ventrally a similar 

 haemal arch in which are two blood-vessels: above, the caudal 

 artery (a continuation of the dorsal aorta) ; below, a caudal vein. 

 Draw the section. 



In the muscles of the tail note the concentric arrangement 

 of the plates in each mass. Cut into the tail portion, splitting one 

 of these masses longitudinally, and determine how this condition 

 is brought about. 



With probe and scissors follow the caudal vein forward to its 

 division into right and left renal portal veins which pass along 

 the dorsal surface of the mesonephros. Note, in this dissection, 

 the iliac arteries passing from the dorsal aorta into the pelvic 

 fins, and, in following these out, see also the iliac veins arising 

 from the fins and passing into the lateral abdominal veins. 



Add these arteries and veins to your drawings, which should 

 now include the chief blood-vessels of the body. 



Spinal Nerves. Remove the roof of the sinus venosus and 

 see, lying upon the muscles, the trunks of the spinal nerves which 

 enter the pectoral fin. How many are there of these? Is there 

 anything in their course to indicate a shifting of position of the 

 fin? Do any of them pass through cartilage to enter the fin? 

 Do you find any cross-connections (a plexus) between any of these 

 nerves of the pectoral region? 



Trace some of these nerves toward the middle line. Do you 

 find any evidence that they are composed of two portions or 

 roots? If so, do you find an enlargement (ganglion) on either 

 root? 



In the pelvic region work out in the same way the nerves 

 which enter the pelvic fin. Dissect the nerves in front of the 

 fin carefully and see that several (how many?) unite in a longi- 

 tudinal trunk (collector nerve) which passes backwards to 

 enter the fin, with others which pass straight out from the median 

 line. 



