COMPARATIVE ANATOMY OF THE MUSCULAR SYSTEM 129 



skin without the further use of the knife. If you attempt to cut the skin from 

 the body by means of a scalpel you will slash into the muscles. After removing 

 a considerable area of skin note that the body wall under the skin is composed 

 of a coat of muscles completely sheathing the body. These are the parietal 

 muscles. Observe that the parietal muscles consist of a series of zigzag myotomes, 

 each separated from its neighbor by a white sheet of connective tissue, the 

 myocomma or myoseptum. In the middle of the side of the body observe a 

 white line running lengthwise. This is the outer edge of the horizontal skele- 

 togenous septum, which intersects the skin at the lateral line. This septum 

 divides the myotomes into dorsal portions— the epaxial muscles— and ventral 

 portions— the hypaxial muscles. On the ventral side note that there is a white 

 partition in the median ventral line. This is the linea alba; it separates the 

 myotomes of the two sides of the body. No muscle ever crosses the dorsal or 

 ventral median lines, and hence all of the muscles are paired. 



Draw from the side a portion of the body to show the myotomes. 



2. The modification of the parietal muscles by the presence of appendages.— 

 Strip off the skin from the bases of the pelvic fins and observe that additional 

 muscles are present in the fins at their bases for the purpose of moving the fins. 

 A mass of muscle springs from the myotomes and is inserted on the fin on both 

 dorsal and ventral sides. Cut through the middle of the dorsal mass, carefully 

 separate the cut ends from the body wall, and note that the myotomes are re- 

 vealed underneath the fin muscles. The dorsal ends of the fin muscles will be seen 

 to spring from the myotomes with which they are continuous, and it is important 

 to note that several myotomes contribute in this way to the fin muscles. We 

 thus learn that in the neighborhood of an appendage the myotomes bud off 

 muscle slips for the appendages. In this manner the muscles of the appendages 

 originally arose. It should be stated, however, that in the higher vertebrates the 

 intrinsic appendicular muscles can no longer be seen to originate in this fashion 

 but are formed in place in the limbs. 



3. The visceral muscles of the dogfish. — Make a median ventral incision in 

 the skin of the ventral side of the head between the gill slits and strip off the skin 

 in an upward direction over the gill slits and up to the median dorsal fine. Note 

 that dorsally above the gill slits the myotomes are present and as typical in form 

 and arrangement as in more posterior parts of the body. In the region of the gill 

 slits and on the ventral surface of the head, however, entirely new sets of muscles 

 are found which serve to move the gill arches and the jaw. These muscles are 

 the visceral muscles, and they are derived from the hypomeres. There are a 

 great many of them, and it is not our purpose to identify them in detail, but 

 the following may be noted : trapezius, the long muscle above the gill slits, and 

 below the myotomes; the dorsal constrictors, between the dorsal portions of 

 the gill arches, and with their fibers directed obliquely forward; the ventral 

 constrictors, between the ventral portions of the gill arches, and covering most of 



