6io 



THE DEPRESSOR NERVE. 



the cricothyroid muscle (while the other laryngeal muscles contract rejiexly). 

 Section of both nerves, owing to paralysis of the cricothyroids, causes slight 

 slowing of the respirations (Sklarek). In dogs, the voice becomes j deeper and 

 hoarser, owing to diminished tension of the vocal cords (Longet). The larynx 

 becomes insensible, so that saliva and particles of food pass into the trachea and 

 lungs, without causing reflex contraction of the glottis or coughing. This excites 

 "traumatic pneumonia," which results in death (Friedldnder). 



Stimulation of the recurrent nerves causes spasm of the glottis. Section 

 of these nerves paralyses the laryngeal muscles supplied by them, the voice becomes 

 husky and hoarse (in the pig Galen, Riolan, 1618) in man, dog, and cat ; while 

 rabbits retain their shrill cry. The glottis is small, with every inspiration, the 

 vocal cords approximate considerably at their anterior parts, 

 while, during expiration, they are relaxed and are separated 

 from each other. Hence, the inspiration, especially in young 

 individuals whose glottis respiratoria is narrow, is difficult 

 and noisy (Legallois) ; while the expiration takes place easily. 

 After a few days, the animal (carnivore) becomes more quiet, 

 it respires with less effort, and the passive vibratory move- 

 ments of the vocal cords become less. Even after a con- 

 siderable interval, if the animal be excited, it is attacked w T ith 

 severe dyspnoea, which disappears only when the animal has 

 become quiet again. Owing to paralysis of the laryngeal 

 muscles, foreign bodies are apt to enter the trachea, while 

 the paralysis renders difficult the first part of the process of 

 swallowing in the oesophageal region. Broncho-pneumonia 

 may be produced (Amsperger). 



6. The depressor nerve, which in the rabbit arises by one 

 branch from the superior laryngeal, and usually also by a 

 second root from the trunk of the vagus itself [runs down 

 the neck in close relation with the vagus, sympathetic, and 

 carotid artery, enters the thorax], and joins the cardiac plexus 

 (fig. 434, sc). It is an afferent nerve, and when its central 

 end is stimulated [provided both vagi be divided], it dimin- 

 ishes the energy of the vaso-motor centre, and thus causes a 

 fall of the blood-pressure (hence the name given to it by Cyon 

 and Ludwig, 371, II.). At the same time, [if the vagus 

 on the opposite side be intact], its stimulation affects the 

 cardio-inhibitory centre, and thus reflexly diminishes the num- 

 P, pons ; M, medulla ber of heart-beats. [Its stimulation also gives rise to pain, 

 oblongata ; Vag, va- s0 that it is the sensory nerve of the heart. If in a rabbit 

 ^inferior laryngeaf; the va S i be divided in the middle of the neck, and the central 

 ac] superior cardiac or en( * of the depressor nerve, which is the smallest of the three 

 depressor ; ic, inferior nerves near the carotid, be stimulated, after a short time there 

 cardiac or cardio-in- j s n0 alteration of the heart-beats, but there is steady fall of 

 nibitory ; H, heart. the blood-pressure (fig. 106), which is due to a reflex inhibi- 

 tion of the vaso-motor centre, resulting in a dilatation of the blood-vessels of the 

 abdomen. Of course, if the vagi be intact, there is a reflex inhibitory effect on the 

 heart. It is doubtful if the depressor comes into action when the heart is over- 

 distended. If it did, of course the blood-pressure would be reduced by the reflex 

 dilatation of the abdominal blood-vessels.] 



The depressor nerve is present in the cat ( 370), hedgehog {Aubert, Rover), rat and mouse ; 

 in the horse and in man, fibres analogous to the depressor re-enter the trunk of the vagus 

 (Bernhardt, Kreidmann). Depressor fibres are also found in the rabbit in the trunk of the 

 vagus (Drcschfchlt, Stelling). 



Vag 



Fig. 434. 

 Scheme of the cardiac 

 nerves in the rabbit. 



