An introduction to physiology . lthe beginning of each period as he marks it onthe drum. At the beginning of the second pe-riod, open the short-circuiting key; at the begin-ning of the third period, close the short-circuitingkey. Lower the drum when one circuit iscompleted. Count the number of beats in each period. Thefrequency will be increased. The force of con-traction will also be The latent periodof excitation is long and there is a prolongedafter-effect. The former frequency is regainedmore rapidly after short than after long stimula-tions. The speed of the cardiac excitation
An introduction to physiology . lthe beginning of each period as he marks it onthe drum. At the beginning of the second pe-riod, open the short-circuiting key; at the begin-ning of the third period, close the short-circuitingkey. Lower the drum when one circuit iscompleted. Count the number of beats in each period. Thefrequency will be increased. The force of con-traction will also be The latent periodof excitation is long and there is a prolongedafter-effect. The former frequency is regainedmore rapidly after short than after long stimula-tions. The speed of the cardiac excitation wave 1 The stimulation of the augmentor fibres is difficult andoften fails in winter frogs. 558 THE OUTGO OF ENERGY (compare page 336) is increased and the time ofits passage across the auriculo-ventricular grooveis shortened, though this cannot be observed bythe method used in the present experiment. The Inhibitory Nerves of the Heart The Preparation of the Vagus Nerve. — Fastena lame fros? on the board, back down. Pass the. Fig. 73. Scheme of the cervical nerves in the frog (after Schenck).G. P. Glosso-pharyngeus. Hg. Hypoglossus. V. Vagus. L. Posterior end of lower jaw. The glosso-pharyngeus has been drawnto one side of the hypoglossus for the sake of clearness. glass tube through the oesophagus into thestomach. Eemove the muscles lying over thepetrohyoid muscle, which passes from the base ofthe skull to the horn of the hyoid bone. Lying INNERVATION OF HEART AND BLOOD-VESSELS 559 near the line between the angle of the jaw andthe auricle are four nerves (Fig. 73) : (1) Thehypoglossals. This nerve is superficial. Neartheir emergence from the skull it is the lowestof the nerves, but later, the uppermost. It crossesthe remaining nerves and the blood-vessels, andpasses forwards and inwards towards the tongue.(2) The glosso-pharyngeus, which soon turns for-wards beneath the hypoglossus parallel to theramus of the jaw. (3) The vagus, and (4) thelaryngeus, the two ly
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