808 NERVOUS SYSTEM OF THE HORSE 



runs forward on the ventral part of the left face of the trachea, and continues in the 

 neck in a position similar to the right nerve. 



It is worthy of note that the left nerve passes beneath the bronchial lymph glands as it 

 winds around the aorta; also that in the next part of its course it hes between the left surface of 

 the trachea and the deep face of the aorta, and is then related to lymph glands which^ he along 

 the ventral aspect of the trachea. The left recurrent is often incorporated in part of its course 

 in the anterior mediastinum with a deep cardiac nerve. Further, the left nerve lies at first ven- 

 tral to, and then upon, the oesophagus in the neck. The right recurrent is given off from, or in 

 common with, a considerable trunk wliich connects the vagus with the first thoracic gangUon 

 of the sympathetic. The arrangement here is commonly more or less plexiform, and from it 

 one or two cardiac nerves arise. 



The terminal part of each nerve (Fig. 649) lies on the dorsal surface of the 

 trachea, in relation to the oesophagus medially and the carotid artery dorsally.^ It 

 passes between the crico-arytenoideus dorsalis and the crico-pharyngeus, and 

 enters the larynx at the medial side of the lamina of the thyroid cartilage. Before 

 entering it gives branches to the crico-arytenoideus dorsalis and arytenoideus 

 transversus, and afterward supplies the internal muscles of the larynx. It also 

 commmiicates by delicate filaments with branches of the anterior larjmgeal nerve. ^ 

 Collateral branches are given off to the cardiac plexus (Rami cardiaci), to the 

 trachea (Rr. tracheales), to the oesophagus (Rr. oesophagei), and to the posterior 

 cervical ganglion of the sympathetic. 



4. Cardiac branches (Rr. cardiaci), usually two or three in number, are given 

 off from each vagus within the thorax (Figs. 553, 554). These concur with the 

 cardiac branches of the sympathetic and recurrent nerves to form the cardiac 

 plexus, which iimervates the heart and great vessels. 



5. Small tracheal and oesophageal branches (Rr. tracheales et oesophagei) 

 are given off from both vagi in the thorax. These concur with branches from the 

 recurrent nerves and the posterior cervical and anterior thoracic ganglia of the 

 sympathetic in forming the posterior tracheal and oesophageal plexuses, from 

 which twigs go to the trachea, oesophagus, heart, and large vessels. 



6. Bronchial branches (Rr. bronchiales) are detached at the roots of the lungs 

 and unite with sympathetic filaments in forming the pulmonary plexuses. From 

 the latter numerous branches proceed in a plexiform maimer along the bronchi 

 and vessels into the substance of the lungs. 



The vagus and glosso-pharyiigeal nerves are so closely associated in origin and central 

 connections that they may be described together in this respect. 



The sensory fibers arise from the petrous and jugular ganglia, and their central parts enter 

 the lateral aspect of the medulla and divide into anterior and posterior branches Uke the fibers of 

 the dorsal roots of the spinal nerves. Most of the fibers end in arborizations about the cells of the 

 vago-glosso-pharyngeal nucleus of termination, which consists of two parts. Of these, the dorsal 

 sensory nucleus (Nucleus ake cinereae) is situated in the posterior part of the floor of the fourth 

 ventricle and in the adjacent part of the closed portion of the medulla near the .median plane. 

 The other part is termed the nucleus of the solitary tract, and is so named because its cells are 

 grouped about the bundle (Tractus solitarius) formed by the posterior divisions of the afferent 

 nerve-fibers. It ends about the level of the pjTamidal decussation. The secondary central con- 

 nections are similar to those of the sensory part of the trigeminus. 



The motor fibers (and those of the medullary part of the accessory) arise from the dorsal 

 motor nucleus and the ventral motor nucleus. The cells of the former lie in groups along the 

 ventro-medial side of the dorsal sensory nucleus. The latter, also termed the nucleus ambiguus, 

 is situated more deeply in the lateral part of the formatio reticularis. 



THE SPINAL ACCESSORY NERVE 



The spinal accessory nerve (N. accessorius) is purely motor. It consists of 

 two parts which differ in origin and function. 



1 The nerve lies here about a finger's breadth above the thjrroid gland. If, however, the 

 gland is enlarged it may come in contact with the nerve. The same is true in regard to the rela- 

 tion to the anterior oer\'ioal lymph glands. 



^ The recurrent nerves innervate all the intrinsic muscles of the larjmx except the crico- 

 thyroid. 



