MAMMALIA. 293 



Numerous small lymphatic glands, which resemble the cervical 

 glands of the pigeon, are present in various situations. Those 

 belonging to the lacteals are known, from their position, as 

 mesenteric glands. 



An elongated flattened spleen, of a dark-red colour, is attached 

 by a flap of mesentery to the cardiac end of the stomach. 



The thymus gland is a fat-like mass, largest in young animals, 

 closely connected with the base of the heart. The thyroid gland 

 consists of two small elongated bodies of reddish colour, closely 

 applied to the sides of the front end of the trachea, and con- 

 nected together by a ventral bridge of tissue. 



New colourless corpuscles are developed in the lymphatic glands, 

 thymus, and spleen, new red corpuscles in the red marrow of 

 bones, and from pre-existing colourless corpuscles. The worn- 

 out red corpuscles are probably broken down in the spleen. 



6. The respiratory organs (Fig. 85) are lungs contained in 

 the thorax, and communicating with the exterior by a trachea, 

 the front end of which is modified into an organ of voice 

 (larynx). 



The slit-like glottis, situated at the base of the tongue, and 

 guarded in front by an elastic flap (epiglottis), leads into the 

 larynx. This is supported by a broad thyroid cartilage (which is 

 a bent plate, incomplete dorsally), and, posterior to this, by a 

 ring-like cricoid cartilage, the dorsal side of which is thickened. 

 Two elastic folds, the weal chords, project from the sides of the 

 larynx. They are attached below to the thyroid cartilage, above 

 to two small arytenoid cartilages, which articulate dorsally with 

 the front edge of the cricoid. The larynx may be regarded as 

 the modified front end of the windpipe or trachea, which runs 

 along the neck ventral to the oesophagus, and is supported by 

 cartilaginous hoops. The trachea bifurcates within the thorax, 

 just anterior to the base of the heart, into two bronchi. 



The lungs are two spongy bodies, pink in colour, which mainly 

 fill the thoracic cavity, as long as its walls remain intact. The 

 left lung is subdivided into two, the right lung into four lobes of 

 very unequal size. Each lung is enveloped by a delicate mem- 

 branous pleura, which consists (like the peritoneum) of a visceral 

 layer, closely applied to the surface of the lung, and a parietal 

 layer, lining one half of the thorax. The two layers are continuous 

 at the root of the lung. 



