CIRCULATION OF THE BLOOD. 



545 



-HB 



-PH 



<— OH 



ganglion or removal of the heart from the influence of the main motor 

 ganglion. 



To carry this subject still further, a more delicate means of experimentation 

 must be used. Poisons must be employed as instruments of investigation. 

 Pharmacology has indeed in this line almost run ahead of physiology, for it has 

 been through the study of the action of poisons on the heart that our complete ideas 

 of cardiac physiology have been derived. 



Fig. 226 represents an apparatus devised by Dr. Coats, of Glasgow, and 

 Professor Ludwig, of Leipsic. It consists of a reservoir, A, with a stop-cock, 

 B, containing fresh serum; a rubber tube, C, leading from this, and a cannula, D, 

 which is to be inserted into the vena cava inferior; another cannula, D', to be 

 inserted in the aorta, connected by tubing with a mercurial manometer, E, i.e., a 

 fine U-shaped tube partly filled 

 with mercury, and supporting on 

 one limb of the column a piston 

 with a long, delicate wire rod, G, 

 above it. 



The brain and spinal cord of 

 a frog should be destroyed by 

 introducing a needle into the cere- 

 bio-spinal canal, the heart freely 

 exposed, and one of the pneumo- 

 gastric nerves carefully dissected 

 out. To find the vagus nerve, 

 follow up the diverging aorta? to 

 where they cross the cartilaginous 

 tips ' of the posterior horns of the 

 hyoid bone : from each of these tips 

 the petro-hyoid muscles are seen 

 passing upward and backward to- 

 ward the occipital region. The 

 lower border of these nearly par- 

 allel fibres is the guide to the vagus, 

 which is found lying beneath its 

 inner edge. Following these 

 muscles back from their insertion 

 in the hyoid bone to their origin in 

 the petrous bone, they are seen to 

 be crossed first by the hypoglossal 

 nerve, ascending inward to the 

 muscles of the tongue. Nearer 

 the middle line and following the 

 same course as the hypoglossal 

 is seen the glosso-pharyngeal, and 

 crossing over the top of the in- 

 ferior horn of the hyoid bone is 

 the laryngeal nerve (Fig. 227). 



Place a thread loosely around the nerve, so that it can be easily found when 

 required. The next step is to insert the cannula, D, into the inferior vena cava, 

 and secure it with a thread ; the cannula, D', is then inserted into one aorta, the 

 other being ligated. All the other organs may be removed, leaving only the 

 thorax, heart, and a large fragment of skin, S, to cover the heart and nerve, to pre- 

 vent drying. The cesophagus is now distended with a large glass rod, firmly 

 clamped to an upright stand. The next step is to connect the vena cava by 

 means of its cannula with the reservoir containing serum. Open the stop-cock 

 for a moment, and allow the serum to pass through the heart and apex of the 

 arterial cannula, to wash out all the blood from the heart. The arterial cannula 

 is then connected with the manometer, and the serum allowed to flow through the 

 heart into the manometer until the air in the proximal is entirely expelled through 

 at F. Then the apparatus is ready for use. The heart should be filled so full that 

 a little tension exists, even during the diastole " ' ' " ' 



35 



Fig 



227.— Diagram of the Course of the 

 Vagus Nerve in the Frog. [Stirling.) 



H. heart; LU, lung; BR, brachial plexus ; HY, hypoglossal 

 nerve; V, vagus; I., laryngeal nerve ; GP. glosso-pharyngeal 

 nerve: SM, submental muscle; GH, genio-hyoid muscle; HG, 

 hyoglossal muscle; HB, hyoid bone; PH, petro-hyoid muscle; 

 OH, omo-hyoid muscle ; SH, sterno-hyoid muscle. 



It will be noticed that at each 



