12 WILLIAM F. ALLEN 



a neural vein. Likewise each alternate ventral intermuscular 

 septum receives a haemal artery, and from every intermediate 

 ventral sejita there comes a haemal vein. Furthermore each 

 alternate lateral intermuscular septum receives a lateral artery, 

 and from every intermedian septa there comes a lateral vein. 

 A glance at fig. 9 demonstrates that one intermuscular septum 

 does not, however, possess a neural, haemal, and two lateral arter- 

 ies, but, on the contrary, that septum may receive, as shown oppo- 

 site vertebra numbered 9 in fig. 9, a neural, a lateral, and a haemal 

 vein and a lateral artery ; while the intermuscular septum opposite 

 vertebra numbered 8 in the same figure has a neural, a lateral 

 and a haemal artery and a lateral vein. Occasionally, however, 

 some irregularities may occur, as for example, opposite verte- 

 bra numbered 10 in fig. 9 and under the posterior vertebrae. 



Of the two classes of arteries arising from the side of the caudal 

 artery, those designated as lateral arteries (figs. 9 and 10, L.A.; 

 indicated but not lettered in fig. 4), after leaving the haemal 

 canal, curve around to the side of the centra, where each bends at 

 right angles to pass laterad in the intermuscular septum. When 

 about half way between the vertebra and the skin it divides into 

 a dorsal and a ventral branch. In their course these branches 

 supply the myotome in front and the one behind, and upon arriv- 

 ing at the surface, above and below the lateral lymphatic trunk 

 they usually follow dorsad or ventrad a short distance on the sur- 

 face of the intermuscular septum to give off branches to the con- 

 nective tissue between the skin and the muscles. 



The other lateral branches of the caudal artery (figs. 4 and 4a, 

 Neu.A.,; figs. 9 and 10, Neu. and L.A.) are larger and more im- 

 portant vessels. Like the lateral arteries described above they 

 curve around to the side of the centra ; where each separates into 

 a lateral and a neural artery. The former (figs. 4 and 4a, L.A.) 

 is identical to the lateral artery described above, and the latter 

 (figs. 4 and 4a, Neu. A.) curves around to the dorsal side of the 

 vertebra, where it follows up along the anterior surface of the 

 corresponding neural spine. In crossing the vertebra it gives 

 off a myelonal artery, which presumably passes through the spinal 

 nerve formamen, to supply the myelon or the spinal cord; this 



