THE ANATOMY OF THE THYROID GLAND 167 



in Raia, and subject in all species to great individual variation in 

 volume. The muscles take origin posteriorly from the coracoid 

 arch, anteriorly from the hyoid arch and mandible, and laterally 

 from the outer surfaces of the branchial arches. Acting from 

 these ''fixed points" upon the more movable, but tough and in- 

 elastic skin, these muscles form a very powerful constrictor of the 

 pharynx and collectively are very properly termed the "con- 

 strictor pharyngis" (fig. 1,A). In addition to this constriction 

 the muscular contraction at the same time tends to draw open the 

 branchial clefts, thus permitting the more ready passage of water 

 during the rhythmic pharyngeal contraction or respiratory move- 

 ment. The muscular fibers of the constrictor pharyngis are inti- 

 mately adherent to the derma. 



of the incisions exposing the margins of the coraco-hyoideus muscle. One [)lade 

 of the scissors is then pushed beneath the skin where it readily passes between 

 the coraco-hyoideus and coraco-mandibularis muscles (fig. 1); the incision is 

 continued across the median line from side to side. This divides the coraco-man- 

 dibularis; its anterior portion is grasped with the forceps, lifted, and a longi- 

 tudinal incision through the skin and fascia carried forward along either margin 

 of the muscle.' In the dogfish the divided muscle with the attached skin is easily 

 raised and the loosely attached deep cervical fascia dissected away from its 

 dorsal surface, exposing the thyroid gland. In Carcharias it is better to dissect 

 the deep cervical fascia away from the ventral surface of the coraco-hyoideus 

 muscle, rather than from the coraco-mandibularis; the thyroid gland is then 

 raised with the latter muscle and dissected out from the mass of connective tissue 

 which envelopes it. Finally, the gland must be dissected away from its anterior 

 attachment to the margin of the basi-hyal cartilage, or, in Carcharias, to a median 

 depression, in the ventral surface of this cartilage, which corresponds to the fora- 

 men caecum linguae of mammals; occasionally this depression is a true foramen, 

 in which case the thyroid process becomes obviously analogous to the lobulus 

 pyramidalisof the mammalian thyroid gland. This lobule is represented in Muste- 

 lus by a short triangular projection, not constantly present, which overlies a 

 shallow median groove in the anterior margin of the cartilage. In Raia a similar 

 condition is much less frequently present. 



The thyroid gland is readily accessible from the oral cavity. A needle passed 

 through the oral mucosa just in front of the basi-hyal cartilage— "lingual bone" 

 —enters the substance of the thyroid gland if directed backward in Mustelus 

 and Squalus, well backward and close to the cartilaginous surface in Carcharias, 

 or backward and slightly ventralward in Raia. A transverse incision through 

 the oral mucosa, parallel to and just in front of the basi-hyal cartilage, exposes 

 the anterior margin of the thyroid gland and it may then be readily dissected 

 out from Mustelus or Raia, though with greater difficulty from Carcharias or 

 Squalus. 



