1383. Cc. H. DANFORTH 
Innervation. The segments are supplied by the ventral branches 
of corresponding nerves. 
Blood supply. The Aa. thoracico-dorsalis et thoracico-ventralis 
and the terminal ends of the segmental arteries supply blood to 
these muscles. It is carried away by the lateral abdominal vein. 
Action. This musculature is accessory to the lateral muscle and 
also tenses the body wall between pectoral and ventral fins and 
tends to increase the intra-abdominal pressure. 
VII. MUSCLES OF THE MEDIAN FINS 
Muscles of the dorsal fin 
As has been stated by Bridge (’96) there are twenty-one radial 
cartilages in the dorsal fin. Corresponding with each of these 
cartilages there are developed two muscles on each side. The 
two muscles on the same side of a ray however are only indis- 
tinctly separated from ‘ach other. The more superficial arises 
on the aponeurosis of the myomere beneath from its dorsal and 
to some extent its mesial aspect. There are two of these muscles 
for each myomere. The first ones arise in connection with the 
twenty-fourth. They are all inserted beneath the horny rays of 
the fin. The deep muscles correspond throughout with the super- 
ficial. They arise from the fascia near the median line and from 
the basalia and radialia of the fin. They are inserted on the 
same structures as the superficial muscles and poetbly also upon 
the pterygiophores. 
Innervation. ‘These muscles are supplied by ihe nerves of the 
myomeres with which they are associated, the nerve from each 
passing in between the two muscles of a pair and giving off its 
fibers anteriorly and posteriorly. There is also developed, at 
least behind, a small longitudinal trunk derived from anastomoses 
of the several nerves. 
Blood supply. Segmental vessels. 
Action. Contraction of these muscles pulls the fin towards the 
side. The posterior muscles, being more oblique and at the 
same time the hind part of the fin being more or less free, there 
_ is more motion permitted behind than in front. 
