en. xx.] INNERVATION OF THE HEART. 247 



of it large enough to admit the heart ; a glass tube is securely fixed into a 

 small opening on the opposite side of the ball. The animal is anaesthetised, 

 and its thorax opened. The animal is kept alive by artificial respiration. 

 The pericardium is then opened by a crucial incision, the heart is slipped 

 into the ball : the pericardium overlaps the outside of the ball, and the 

 apparatus is rendered air-tight by smearing the edges of the hole with 

 vaseline. The four corners of the pericardium are then tightly tied by 

 ligatures round the glass tube just mentioned. This tube is connected by a 

 stout india-rubber tube to a Marey's tambour or a piston -recorder, the 

 writing point of which is applied to a moving blackened cylinder. When 

 the heart contracts, air will be withdrawn from the tambour to the cardio- 

 meter ; when the heart expands, the air will move in the reverse direction. 

 These movements are written by the end of the lever of the tambour, and 

 variations in the excursions of this lever correspond with variations in the 

 amount of blood expelled from or drawn into the heart with systole and 

 diastole respectively. By calibrating the instrument the actual volume of 

 the blood expelled can be ascertained. 



This instrument has to a great extent replaced a more elaborate cardio- 

 meter invented by the late Prof. Roy. His instrument was made of metal, 

 and oil instead of air was used as the medium in its interior. 



Innervation of the Heart. 



The nerves of the heart, which under normal circumstances 

 control its movements, are the following : 



1. Cardiac branches of the vagus. 



2. The cardiac branches of the sympathetic. 



3. The intrinsic nerves of the heart. These were formerly 

 regarded as more or less independent of the other two sets of 

 fibres ; we now know, however, that they are merely the termi- 

 nations of the other nerves in the heart-wall. For convenience 

 of description, however, we will keep the old name. 



The Vagus. This arises from the grey matter in the floor 

 of the fourth ventricle, at the point of the calamus scriptorius. 

 It leaves the bulb by some 10 15 bundles behind the ninth 

 nerve, and leaves the skull by the jugular foramen, having upon 

 it a ganglion called the jugular ganglion,. Shortly afterwards it 

 s through a second ganglion called the ganglion trunci vagi. 

 It gives off branches to the vessels of the meninges and to the 

 ear, and then receives certain connecting branches : (a) from the 

 glosso-pharyngeal ; and (6) it receives the whole inner division 

 of the spinal accessory nerve. This nerve arises from a centre 

 in the bulb close to and below the vagal nucleus ; the outer 

 half of the same nerve arises from spinal roots, and supplies 

 the sterno-mastoid and trapezius. 



The fibres of the spinal accessory that join the vagus are 

 chiefly motor, especially to the larynx ; some are cardio-inhibitory 

 (see p. 252). 



The vagus then gives off branches to the pharynx, larynx, 



