208 THE VOYAGE OF H.M.S. CHALLENGER. 



described as being inserted into the thyroid cartilage. I regard that insertion as belong- 

 ing to the contractor tracheae rather than to the sterno-trachealis. At the same time, in 

 as much as some of the fibres composing the sterno-trachealis are directly continuous 

 with those of the contractor tracheae, the former muscle may not improperly be described 

 as being inserted into the thyroid cartilage, although its principal and direct insertion 

 is undoubtedly, as above described, into the tube of the trachea. 



3. Contractor trachece. 

 Broncho-trachealis, Owen, Anatomy of Vertebrates, vol. ii. p. 223. 



Attachments.— The muscle so named by MacgUlivray (History of British Birds, vol. ii. 

 p. 25) arises from the posterior border of the inferior surface of the thyroid cartilage, and 

 may be considered as a prolongation backwards of the thyro-hyoid muscle, with which it 

 is almost continuous. From this origin it extends along the lateral border of the trachea, 

 and terminates in a pointed extremity which is inserted into the lateral surface of that 

 tube, -| an inch in front of its bifurcation. As the muscle passes backwards its 

 fibres are attached to each of the tracheal rings. Its posterior extremity extends 

 beyond the insertion of the sterno-tracheal muscle, with which some of its fibres are con- 

 tinuous, and although it does not reach the syrinx, it evidently represents the single 

 muscle (broncho-trachealis) with which, according to Cuvier (vol. iv. p. 473), that organ 

 is provided in the genera Falco, Larus, Phalacrocorax, &c. 



Action.- — This muscle when contracting approximates the tracheal rings to one 

 another and shortens the tube. 



Relations. — The origin of this muscle is situated below (in front of) the insertion of 

 the cleido-trachealis, and corresponds to the origin of the thyro-hyoid muscle. Between 

 the muscles of opposite sides there is an interval free of muscle fibres, both on the superior 

 and inferior surfaces of the trachea, in which the rings of the trachea are seen uncovered 

 by muscle. 



Nerve supijly {V) 



Variations. — (PI. XIX.) — In Eudyptes chrysocome from Kerguelen, the two muscles 

 of ojjposite sides are in contact along the upper surface of the trachea for the 

 posterior half of that tube. There is thus no interval between the two as in Eudyptes 

 chrysocome from Tristan d'Acunha. In Eudyptes chrysocome from the Falklands, the 

 insertion of the contractores tracheae is situated f of an inch in front of the tracheal 

 bifurcation. 



In Eudyptes chrysolophus, in which the trachea is larger than in Eudyptes 

 chrysocome, the contractor tracheae is likewise inserted f of an inch in front of the bifur- 

 cation. Consequently, the muscle in this species is relatively to the trachea longer than 

 in Eudyptes chrysocome. 



