166 JEREMIAH S. FERGUSON 



tral wall of the pharynx and contains the whole course of the ventral 

 aorta and its immediate branches. In Raia the heart is contained 

 within this area at its posterior margin, Ijang in the median line 

 just in front of the cartilaginous arch, but in Mustelus, Squalus 

 and Carcharias the heart has been pushed backward and lies 

 just beneath the coracoid arch. 



The skin of the ventro-cervical region is thin but very tough; 

 laterally it is folded upon itself at the branchial clefts, on the inner 

 surface of which it becomes continuous with the mucosa covering 

 the gills. The fibers of the mylohyoid and geniohyoid muscles 

 are attached to the derma over the greater part of the ventral 

 cervical area. These muscles form a thin sheet, thickest and most 

 prominent in Carcharias, thinnest and frequently almost wanting 



The first method is the more applicable in Raia, where the skin is loosely 

 attached and the coraco-mandibular muscle is small, thin and easily lifted. With 

 some variations I have followed the method outlined by Lombard (*09). A 

 longitudinal incision is made in the median line through the skin and membranous 

 constrictor pharyngis. The skin and adherent muscle are dissected away and 

 retracted laterally, exposing the coraco-mandibularis. (Fig. 1 B."* A probe is 

 •passed beneath the muscle which, after being well freed, is divided midway 

 between the mandible and the coracoid arch. The anterior flap is grasped and 

 reflected forward, at the same time dissecting away from its dorsal surface the 

 deep cervical fascia in which the thyroid gland is embedded. The gland is easily 

 recognized by its deep yellowish orange color and its peculiar rounded or triangu- 

 lar form. In those exceptional instances when the thyroid gland is displaced for- 

 ward in Raia the anterior division of the coraco-mandibularis will have to be 

 reflected forward all the way to its mandibular insertion before the gland is 

 fully exposed; ordinarily the organ will be found directly over the aortic bifurca- 

 tion about midway between the mandible and the point at which the muscle was 

 bisected. 



The above method is less easily applied to the Selachii for the reason that the 

 skin and the constrictor pharyngis are much more firmly adherent to the under- 

 lying structures than in Raia; moreover, in reflecting forward the anterior divi- 

 sion of the coraco-mandibularis one is almost certain to injure the thyroid sinus, 

 deluging the part with blood, before the gland can be exposed. In Carcharias 

 one encounters the added disadvantage that the thyroid gland is quite firmly 

 united to the coraco-hyoideus and the surface of the basi-hyal cartilage, and the 

 gland is buried in a mass of connective tissue by which it usually is entirely ob- 

 scured even after the coraco-mandibularis has been completely reflected away 

 from its surface. The second method is, therefore, the more applicable in Mus- 

 telus and Squalus and is very much more certain in Carcharias. One blade of 

 a blunt scissors is inserted into the first branchial cleft on the right and then on 

 the left side and the clefts lengthened to their extreme ventral limits, the ends 



