130 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



the ventral surface, extending forward to the mouth, with fibers directed back- 

 ward; and the adductor mandibularis , the thick muscle at the angle of the jaws, 

 used to close the lower jaw. The constrictors open the gill slits by diminishing the 

 distance between the gill arches by their contraction. These muscles of the 

 dogfish persist in higher vertebrates after the gills have been lost in association 

 with those structures which are derived from the gill arches, that is, the jaws, the 

 hyoid, and the cartilages of the larynx. 



C. THE MUSCLES OF NECTURUS 



Animals that have been preserved for some time in formalin are preferable for 

 the study of the muscles. Make a median dorsal incision extending the length 

 of the head and trunk. Loosen the cut edges of skin with the fingers, noting in 

 the cut surfaces the flask-shaped cutaneous glands which secrete slime. Then 

 with the fingers separate the skin from the muscles in a ventral direction until you 

 have removed the skin from the head, trunk, and appendages. The gills are to be 

 left in place. The white fibrous material between the skin and muscles is the 

 subcutaneous connective tissue or superficial fascia. 



1. The muscles of the trunk and tail. These muscles preserve the generalized 

 arrangement typical of primitive and embryonic vertebrates. They consist, as 

 in the dogfish, of a series of myotomes, separated by myosepta. The myotomes 

 are long, nearly rectangular blocks extending from the mid-dorsal to the mid- 

 ventral line. Their narrowed dorsal ends slant forward. Note their division 

 into epaxial and hypaxial portions by the horizontal septum. Although the 

 trunk muscles appear to be unmodified, they are in reality already separating 

 into layers. On cutting into the epaxial muscles they will be seen to constitute 

 a mass whose fibers are all directed forward; this mass corresponds to the lon- 

 gissimus dorsi muscle of higher forms. On cutting into the hypaxial muscles, on 

 the other hand, they will be found divisible into three distinct layers. The 

 outer layer, or external oblique muscle, is composed of fibers directed obliquely 

 ventrad and caudad. The middle layer, or internal oblique muscle, is composed 

 of fibers directed craniad and ventrad. On cutting through the internal oblique 

 a third layer the transverse next to the body cavity will be found. On either 

 side of the median ventral line the fibers of the myotomes are directed parallel 

 to the longitudinal axis of the body and form a narrow band, the rectus abdominis 

 muscle, which is rather distinct just anterior to the pelvic girdle. 



2. The muscles of the pelvic girdle and hind limb. In this region of the 

 body, as in the case of the dogfish, the series of myotomes is interrupted by 

 the presence of limb muscles, extending from the trunk into the limb. Examine 

 the ventral face of the pelvic girdle. It is covered by muscles which may be sepa- 

 rated into two muscles. The anterior muscle covering the pubic cartilage extends 

 from the median line of the girdle to the femur; it may be named the pubofemora- 

 lis externus. The posterior muscle extends from the ischial region of the girdle 



