VOICE AND SPEECH. 599 



by rapidly bringing the shells together. The molluscs provided with shells possess 

 strong retractors. 



In the worms likewise the integument forms with the muscles a musculo- 

 cutaneous tube. The unstriated muscle-fibers pass either longitudinally only 

 (round-worms) , or longitudinally and transversely (scratching worms) , or finally 

 longitudinally, transversely and vertically through the body (flat-worms). Some 

 worms possess muscular suckers, and others one or two pairs of motile stump- 

 like feet. In round-worms the epidermal cells, and in some bristle-worms the 

 intestinal epithelium, pass directly over into muscle-cells, both together being called 

 "epithelio-muscular cells." In the echinoderms also the muscles are united with 

 the integument; in the holothurians there is an external, continuous layer of 

 circular fibers, beneath which is a longitudinal musculature, arranged in five 

 separate bands. 



In the star-fish and the hair-stars special muscles move the limbs of the 

 radiating parts of the body. The sea-urchin, surrounded by a firm lime-capsule, 

 has special muscles that move its spines, and by means of which it is capable of 

 locomotion. The ambulacral feet also aid in locomotion. 



In the celenterates the muscle-fibers are transformed sections of epithelial 

 cells. Hence, there are present "epithelio-muscular cells," which are striated in 

 the medusa, and unstriated in the anemone and hydroid polyp. The free epithelial 

 part may be provided with cilia. In the medusa these elements lie partly on 

 the umbrella and partly on the tentacles. Among the polyps, the actinia have 

 a strong muscular base, and, in addition, longitudinal and circular fibers on the 

 body and on the tentacles. In some polyps muscles also accompany the gastro- 

 vascular apparatus. 



Among the protozoa, striated muscle-fibers have been found in some infusoria, 

 for example in the pedicle of the vorticella; while, in addition, the movements are 

 executed by the movable protoplasm of the body, or by voluntarily motile cilia. 



VOICE AND SPEECH. 



SCOPE OF THE VOICE. PRELIMINARY PHYSICAL CONSIDERATIONS 

 CONCERNING THE PRODUCTION OF SOUND IN REED-APPARATUS. 



The current of expired air, and under certain circumstances also 

 that of inspired air, can be employed to throw the tense true vocal bands 

 of the larynx into regular vibration, as a result of which a sound is pro- 

 duced. This is termed the human voice. 



The true vocal bands of the larynx are elastic, "membranous reeds." By 

 "reeds" are meant elastic plates that almost completely fill the space (frame) 

 in which they are spread out, leaving, however, a small space for their movement. 

 If air be blown against the reeds from a tube below them (air-tube), they will 

 yield at the mcment that the tension of the air overcomes the elastic tension of 

 the reeds. In this way a considerable quantity of air suddenly escapes, its tension 

 rapidly diminishes, and the reeds return to their former position, to repeat again 

 the movement described. From the foregoing it results that 



1. During the vibration of the reeds, alternate condensation and rarefaction 

 of the air must take place. It is chiefly this that (as in the siren) produces the 

 sound, not so much the reeds themselves. 



2. The "air-tube," which conducts the air to the membranous reeds, consists 

 in the human voice-apparatus of the lower section of the larynx, the trachea, 

 and, below, the entire bronchial tree. The bellows is the thorax, diminished in 

 size during expiration by muscles. 



3. The air-passage above the reeds is called a "reinforcing tube," and consists 

 of the upper section of the larynx, the pharynx, and also the oral and nasal cavities, 

 which are arranged in two stories one above the other, and can be closed alternately. 



The pitch of the tone depends upon the following factors: 



(a) The length of the elastic plates. The pitch is inversely proportional to 

 the length of the elastic plates; that is the fewer the units of length that enter 

 into the elastic plates the more numerous will be the units of time (vibrations) 

 entering into the tone produced. For this reason the pitch of the shorter vocal 

 bands in children and in women is higher than that in adults and in men. 



(6) The pitch of the tone is, further, directly proportional to the square root 



