G90 NERVOUS SYSTEM OF THE HORSE 



arteriosum, winds around the concavity of the aortic arch from without inward, 

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

 neck in a similar position 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 lies between the left surface of 

 the trachea and the deep face of the aorta, and is then related to lymph glands which lie 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 

 ventral to, and then upon, the a-sophagus in the neck. The right recurrent is given ofT from or 

 in common with a considerable trunk which connects the vagus with the first thoracic ganglion 

 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. 517) lies on the dorsal surface of the trachea, 

 in relation to the oesophagus internally and the carotid artery above. ^ It passes 

 between the crico-arytenoideus posterior and the crico-pharyngeus; the terminal 

 branches supply all the muscles of the larynx except the crico-thyroid, and communi- 

 cate Avith branches of the superior laryngeal nerve. Collateral branches are given 

 off to the cardiac plexus (Rr. cardiaci), to the trachea (Rr. tracheales), to the 

 oesophagus (Rr. a?sophagei), and to the inferior 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. 428, 429). These concur with the 

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

 plexus, which innervates the heart and great vessels. 



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

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

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

 sympathetic in forming the posterior tracheal and cesophageal plexuses, from 

 which twigs go to the trachea, crsophagus, 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 manner along the bronchi and 

 vessels into the substance of the lungs. 



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

 connections that they may be described together in this respect. 



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

 the lateral aspect of the medulla and divide into anterior and posterior branches like 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 ala3 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 me- 

 dian 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 aiferent nerve-fil)ers. It (>nds about the level of the i:)yramidal decussation. The secondary 

 central coimoctioiis arc snnilar to those of the sensory part of the trigeminus. 



The motor fibers (and those of the medullary part of tlie accessory) arise from the dor- 

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

 the ventro-medial side of the dorsal sensory nucleus. Tlie latter, also termed the nucleus am- 

 biguus, is situated more deeply in the lateral part of the forniatio reticularis. 



THE ELEVENTH OR SPINAL ACCESSORY NERVE 

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

 two parts whicii dilTer in origin and fimction. 



The medullary part arises from the lateral aspect of the medulla by several 

 rootlets which an^ behind and in series with those of the vagus (Fig. 499). The 

 spinal part arises from Ww cervical part of the spinal cord by a series of fasciculi 



' The nerve lies here about a finger's breadth above the thyroid 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 cervical lymph glands. 



