K i; S P 1 U A T ION 1 N T UK C H E L MA. 1<) 



and the sixth to those fibres connecting the central tendon and carapace. Section 

 of the medulla spinalis in the cervical region effectually intercepts communication 

 between these nerves and their usual source of excitation. Under these circum- 

 stances the muscles of the trunk remain at rest, although the movements of the 

 glottis indicate that the creature feels the respiratory need. These glottic move- 

 ments continue normal even after the further section of both pneumogastrics. 



The respiratory nerve of the larynx, the par vagum, emanating from the medulla 

 oblongata, passes out of the cranium at the posterior jugular foramen, and courses 

 down the neck within the sheath of the cervical vessels. Soon after leaving the 

 skull, it gives off the superior laryngeals, and low down in the neck, opposite the 

 aorta, the inferior laryngeal, the two branches that interest us at present. 

 The superior laryngeal (big. 4, &), soon after separating from the parent nerve, 

 approaches the major cornu of the hyoid bone, and under shelter of its pos- 

 terior border, follows it closely to its junction with the body, then winding 

 spirally forwards, it crosses the articulation, and runs along the margin of the 

 excavation in close proximity with the larynx. In this position it gives off three 

 principal branches. 1st. A communicating branch (Fig. 4, V); 2d. A branch to 

 the crico-arytenoid, or opening muscle of the glottis 

 (Fig. 4, &"); and 3d. A branch to the crico-hyoid or 

 glottic sphincter (Fig. 4, &'"). The communicating 

 branch (Fig. G) is a relatively large nerve, but has 

 hitherto escaped observation ; it is easily brought into 

 view by dividing the trachea and lifting it forwards. It 

 passes beneath the larynx directly from side to side, 

 traversing the membrane of the cricoid fenestrum, about 

 its middle. It is composed of fibres derived in part from 

 each of the superior laryngeal nerves, which cross each pj g . g. The intercommunicating 

 other, to be distributed to the glottic muscles of the side ^ve seen from beiow.-a «' ,m- 



. . „, . perior laryngeal nerves; a", inter- 



opposite to that from winch they Originate. ihlS re- communicating nerve crossing the 



markable nerve, we believe, furnishes the only known fenestrum of the cricoid cartilage; 



c • i i • b, crico-arytenoid muscle ; c, crico- 



instance m nerve anatomy of an extracranial chiasm. hvoidmU g Cle . 



Some few filaments penetrate the cricoid membrane, to 



be distributed to the mucous membrane of the larynx, and are doubtless sensitive 



fibres. At page 20 of the physiological section, will be found the experiments by 



which we have determined the function of this intercommunicating nerve. 



The second and third branches present no peculiarities; they penetrate the 

 muscles and are lost to view. Sometimes, however, they can be seen to divide into 

 three or more filaments before so doing. 



Pig, 4^ c . — The pneumogastric, before reaching the aorta, gives off a branch, 

 which, winding around the arch, changes its course upwards, and soon divides 

 into two nerves; one crossing the neck enters the oesophageal tissue— the other, 

 the recurrent laryngeal (Fig. 4. c), joins the trachea, and, in close contact with 

 its side, follows it to the larynx, and enters the crico-arytenoid muscle. There 

 are no fibres from the recurrent distributed to the crico-hyoid directly, or indirectly 

 through communication with the superior laryngeal. 



