468 RESPIRATORY SYSTEM OF THE DOG 



The true vocal cords are large and prominent. The entrance to the large lateral 

 ventricle and saccule is a long slit parallel with the anterior margin of the true 

 vocal cord. 



The crico-thyroid muscle is thick. The hyo-epiglotticus is well developed, 

 and is double at its hyoid attachment. The anterior part of the thyro-arytenoideus 

 arises on the cuneiform cartilage. Hence Lesbre has suggested the name cuneo- 

 arytenoideus for it. 



THE TRACHEA AND BRONCHI 



The trachea is very slightly flattcnied dorsally. It contains forty to forty-five 

 U-shaped rings; the ends of the rings do not meet dorsally, so that here the wall of 

 the trachea is membranous and is composed of a layer of transverse smooth 

 muscle-fibers outside of the rings, the fibrous membrane, and the mucous membrane. 



The stem bronchi diverge at an obtuse angle, and each divides into two branches 

 before entering the lung. The anterior branch goes to the apical and cardiac 

 lobes, the posterior one to the diaphragmatic lol)e, supplying also the mediastinal 

 lobe of the right lung. The anterior branch of the left bronchus crosses under the 

 pulmonary artery. 



THE LUNGS 



The hmgs (Figs. 342, 343) differ in shape from those of the horse and ox in 

 conformity with the shape of the thorax, which is relatively very wide in the dog; 

 the lateral thoracic walls are strongly curved, and the costal surface of the lungs 

 is correspondingly convex. There is no cardiac notch on the left lung, although 

 a small part of the pericardium is not covered by the lung at the lowest part of 

 the fifth and sixth intercostal spaces. On the right side the cardiac notch is 

 opposite the fourth and fifth intercostal spaces. Each lung is divided by deep 

 fissures into three lobes — apical, cardiac, and diaphragmatic ; the right lung has 

 in addition a mediastinal lobe. The apical lobe of the right lung is considerably 

 larger than that of the left. The latter is often more or less fused with the cardiac 

 lobe. Variable accessory lobes may be found. On account of the small amount of 

 interlobular tissue the lobulation is not distinct. 



THE THYROID GLAND 



The lateral lobes of the thyroid gland are long and narrow, and have a flat- 

 tened, ellipsoidal form ; they are placed on the lateral surfaces of the trachea near the 

 larynx. The extremities are small, the posterior one often being pointed. The 

 isthmus is inconstant and variable; in large dogs it usually has the form of a glandu- 

 lar l)and which may be nearly half an inch (ca. 1 cm.) wide; in small dogs it is 

 usually absent. 



Accessory thyroids are freciuently present; three or four nui\' be found on 

 either side, as well as a median one near the hyoid bone. 



The parathyroids, about the size of millet or hemp seed, are four in number 

 usually. Two are ])laced on the deep face of the thyroid lol)es; the others are 

 placed laterally near the anterior extremity of the thyroid. 



THE THYMUS 



The thymus is relatively small, and is situated almost entirely in the thorax. 

 The left lobe is much larger than the right, and extends back as far as the sixth 

 rib at the time of its greatest size. According to Baum the gland increases in size 

 during the first two weeks after birth, and atrophies rapidly in the next two or 

 three months. Traces of it are present at two or three years of age, and may be 

 found even in old subjects. 



