204: AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



arteries, which subsequently contract, and continue the pumping 

 work during diastole, so that jerking is avoided. (2) The blood- 

 supply to any part can be regulated. Owing to the influence of 

 the sympathetic system the muscle is usually kept in a semi- 

 contracted (tonic) state. The same system can cause them to 

 contract more or to contract less, thus diminishing or increasing 

 their calibre. 



The thin walls of the capillaries readily allow the blood-plasma 

 to soak out of them, and waste products to enter them in the 

 tissues, and leave them in the excretory organs. 



The lymph-hearts pump lymph into the blood-system. Blood- 

 plasma exuding in excess through the capillaries is thus carried 

 back again, and some of the products of digestion also enter the 

 blood in this way. 



6. Kespiratory Organs (Fig. 51). The opening of the glottis 

 leads into a small larynx, which is firmly fixed between the 

 posterior hyoid cornua. Its walls are strengthened by two 

 triangular arytenoid cartilages bounding the glottis, and below 

 these by a ring-like cricoid cartilage with several processes. 

 Projecting from the sides of the cavity into its interior are two 

 elastic folds, the vocal cords, one on each side. The larynx 

 opens behind into the two lungs, which lie freely in the dorsal 

 part of the thoracic region. Each is an elongated thin-walled 

 bag, which first dilates and then tapers to a smooth tip. Its 

 inner surface is raised into a prominent series of ridges, which 

 form a close honeycombing. 



The skin must also be considered as a respiratory organ. 



The larynx is lined by a continuation of the ciliated epithelium 

 of the mouth-cavity, and the lungs, for the most part, by simple 

 squamous epithelium. The basis of these organs is fibrous 

 connective tissue, with many elastic fibres and a good deal of 

 unstriated muscle. A network of capillaries immediately under- 

 lies the epithelium. 



Respiration is said to be " buccal," because inspiration (i.e., 

 intaking of air) is effected by the agency of the mouth-cavity, as 

 follows : The mouth being shut, and the walls of the gullet 

 contracted, appropriate muscles lower the floor of the mouth, and 

 thus cause air to rush in by the nostrils. These are then closed, 

 the floor of the mouth is elevated, and air is thus forced into the 

 lungs. Expiration is mainly brought about by the elasticity of 



