248' AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



tract at the same time and pull the chordae taut. The semilunar 

 valves only allow blood to pass out of the ventricles. 



II. The lymph system mainly differs from that of the frog 

 (p. 200) in the absence of lymph-hearts, and the greater definite- 

 ness of the lymphatic vessels {lymphatics of general body, lacteals 

 of gut). These resemble small veins in structure and ultimately 

 open into two delicate tubes, the thoracic ducts, lying just beneath 

 the vertebral column in the thoracic region, and communicating 

 with the veins at the junction of the jugular and subclavian on 

 each side. Minute lymph-spaces are found in all parts of the 

 body, and there are also large lymph-spaces of which the most 

 important is the cceloxn, which includes the general body-cavity 

 and the pericardial cavity. 



Two cervical lymphatic glands are found at the base of the neck, 

 and a small red ovoid spleen is attached to the right side of the 

 proventriculus. 



6. The respiratory organs (Fig. 70) consist of lungs, to which 

 the air gains access through a trachea, and of accessory structures. 

 An organ of voice results from modification of part of the air- 

 passages. 



The glottis leads into a small larynx, the walls of which are 

 supported by several cartilages, and which is the commencement 

 of the windpipe or trachea, a long tube which runs along the 

 neck to the thorax. Numerous firm rings surround and support 

 the trachea, which bifurcates in the thorax into a right and a left 

 bronchus, one going to each lung. At the point of bifurcation is 

 situated the syrinx, which is here the organ of voice. 



The end of the trachea and commencements of the bronchi enclose the 

 tympanum or syringeal cavity. Into this an elastic fold, the membrana 

 semilunaris, projects from the point of bifurcation, and is supported by a 

 slender bar of cartilage, the pessulus, running dorso-ventrally. The inner 

 side of each bronchus, at its commencement, is membranous, and forms the 

 membrana tympani/ormis interna. Special muscles are connected with the 

 syrinx. 



The lungs are spongy bodies, of dark-red colour, which lie in 

 the anterior part of the thorax. They are closely attached to 

 the ventral side of the vertebral column and ribs, and a thin 

 membrane, the pleura, continuous with the peritoneum, covers 

 their ventral surface. The bronchus which runs to each lung 

 enters it, becomes membranous, and, after dilating into a vestibule, 

 runs towards the external side of its posterior end, bifurcating 



