542 BESPIEATORY APPARATUS IN MAMMALIA. 



tion, the epiglottis is shorter, wider at the base, and more triangular than in the other species; 

 the lateral ventricles are sliallow. (There is an interarticular cartilage, as in the Pig ; there is 

 no sub-epiglottidean ventricle, and the vocal cords appear to be nearer each other. The trachea 

 has about forty-two rings, the extremities of which do not meet ; the space between them is 

 less in the Cat than the Dog.) 



The larynx and trachea of Man will be compared with that of animals, when we come to 

 describe the lungs. 



The Thorax (Figs. 255, 322, 323, 324). 



The thorax — also called the thoracic or pectoral cavity — lodges not only the 

 lungs, but also the heart and the large vessels that arise from or pass to that 

 organ, with a portion of the oesophagus and trachea ; as well as nerves, which 

 are as remarkable for their number as their physiological importance. 



Situation. — We have seen that the thorax has for its base the bony cage 

 formed by the ribs, sternum, and bodies of the dorsal vertebrae. Suspended 

 beneath the middle portion of the spine, this cage is transformed into a closed 

 cavity by the intercostal muscles, which fill the spaces between the ribs ; and by 

 the diaphragm, that vast oblique partition which separates the thora.K from the 

 abdomen. 



Internal conformation. — Considered as a whole, the thoracic cavity represents 

 a hollow cone placed horizontally, depressed on each side, and particularly in 

 front towards the summit ; with its base, formed by the diaphragm, cut very 

 obliquely, in consequence of the direction taken by that muscle. This obliquity 

 of the diaphragm renders the antero-posterior diameter of the cavity much 

 greater above than below ; the difference is more than double. 



The internal surface of this conical cavity may be divided into six regions : 

 a superior, inferior, and tivo lateral planes, a base, a posterior plane, and & 

 summit. 



The superior plane presents, on the middle Ime, a large projection resulting 

 from the union of the vertebral bodies ; and, laterally, two deep channels or 

 furrows — the vertehro-costal channels. These latter, wider behind than before, 

 are formed by the upper ends of the ribs ; they lodge the superior border of the 

 pulmonary lobes. The middle projection, or ridge, is comprised between these 

 two lobes. Covered in front by the posterior extremity of the longus colli, this 

 ridge is in relation with, for the remainder of its extent, the posterior aorta, the 

 thoracic duct, and the vena azygos ; on its sides are seen the subdorsal branches 

 of the great sympathetic nerve. 



The inferior plane, much shorter than the preceding, is, like it, narrower in 

 front than behind ; it has for base the superior face of the sternum, the sternal 

 cartilages, and the triangularis sterni muscle. Posteriorly, it gives attachment to 

 the fibrous sac containing the heart. 



The lateral planes, more extensive than the other two, are concave in both 

 their diameters. Formed by the internal face of the ribs and the deep intercostal 

 muscles, they are in contact with the external face of the lung, 



The base, or posterior plane, formed by the convex face of the diaphragm, is 

 circumscribed on its exterior contour by the circle of asternal cartilages, and by 

 the last rib. In it we see the three openings in the diaphragm. 



The summit, or entrance of the thorax, is an oval opening, elongated vertically, 

 comprised between the two first ribs and the longus colli muscle, and which is 

 partly closed by an enormous collection of lymphatic glands ; through this 



