nerv 



AVES. 



301 



es to the dentations along the edge of the 

 mandible. The external branch recedes from 

 the internal, perforates the jaw, and is dis- 

 tributed on its external surface beneath the 

 tegumentary or horny substance which sheaths 

 the extremity of the mandible. It supplies no 

 gustatory branch to the tongue, which is an or- 

 gan of prehension, not of taste, in Birds. 



The facial nerve, or portio dura, exists in 

 Birds, but it is extremely small, its offices 

 being hardly required, in consequence of the 

 structure of the parts of the face in this class. 

 However, a few branches may, with difficulty 

 indeed, be traced, and the trunk of the nerve is 

 constantly present. 



The auditory nerve, or portio mollis, is large, 

 very soft and pulpy, and of reddish colour ; 

 it is received into a deep depression on the 

 internal surface of the cranium (at 7, fig. 137), 

 whence it penetrates by several small foramina 

 to the labyrinth. 



The pneumogastric nerve, or nervus vagus, 

 generally passes out of the cranium in two or 

 three filaments, which afterwards rejoin. On 

 leaving the skull, this nerve communicates 

 with the lingual and glosso-pharyngeal nerves, 

 and is situated between them, the lingual being 

 placed in front. Each nerve of the par vagum 

 passes as a distinct strong cord along the neck 

 in company with the jugular vein, and de- 

 scending into the chest forms the cardiac and 

 pulmonary plexuses, as in Mammalia. The 

 two nerves unite behind the hearl, and proceed 

 along the oesophagus to terminate in anasto- 

 moses with the great sympathetic nerve. 



The gtosso-pharyngeal nerve of the eighth 

 pair passes out of the cranium through the 

 foramen behind the ear, which corresponds to 

 the foramen lacerum posterius, by two filaments, 

 which immediately unite to form an elongated 

 quadrangular ganglion ; this sends off a small 

 internal branch in front of the muscles of the 

 neck ; a small posterior twig which unites with 

 the par vagum, and a large inferior branch to 

 the anterior part of the neck. The latter is a 

 continuation of the nerve itself; it descends 

 along the oesophagus and divides into two prin- 

 cipal branches, of which one passes upwards 

 to the muscles of the os hyoides, between 

 which it is included, and this branch is re- 

 markably tortuous in the Woodpecker in order 

 to be accommodated to the extensile motions of 

 the tongue. The other branch descends along 

 the lateral parieles of the oesophagus, and 

 sends off a twig to join the lingual nerve. 

 The termination of the glosso-pharyngeal is 

 expanded upon the oesophagus. 



The hypoglossal nerve (9th pair) escapes 

 from the cranium posterior to the nervus vagus 

 by the condyloid foramen. It is very slender 

 at its origin; passes to the front of the nervus 

 vagus, partly uniting with, as it crosses over this 

 nerve, and in that situation it detaches a small 

 filament analogous to the descendens noni, 

 which accompanies the jugular vein to the 

 chest. The trunk of the hypoglossal next 

 crosses the glosso-pharyngeal nerve, then passes 

 beneath the cornu of the os hyoides, and ad- 

 vances towards the superior larynx, where it 



terminates by dividing into two principal 

 branches, which are distributed, the one to the 

 anterior and inferior, the other to the superior 

 and internal parts, of the tongue. 



Spinal nerves, These correspond in number 

 to the vertebra- of the spine. They arise, as in 

 the other vertebrata by two roots, the ganglion 

 on the posterior of which is proportionally very 

 large. In the sacral region of the spine, the 

 anterior and posterior roots escape by distinct 

 foramina, and can be separately divided with- 

 out laying open the bony canal, but they are 

 deeply seated and well protected by the anchy- 

 losed processes of the sacrum and the extended 

 iliac bones. 



The cervical nerves vary considerably in 

 number, the known extremes being from ten 

 to twenty-three, corresponding to the number of 

 vertebrae. They are proportionally larger than in 

 man, are tortuous in their course, to be accom- 

 modated to the extensive motions of the neck, 

 and are principally lost in the integument. 

 Only the last, or last two, pairs (u' u", fig. 134^) 

 of cervical nerves concur in the formation of 

 the brachial plexus, which is completed by the 

 first two pairs of dorsal or thoracic nerves ( v). 



The dorsal nerves do not present any notable 

 differences from those of mammalia. 



The sacral nerves have no other peculiarity 

 than their mode of passing out of the spinal 

 canal: they form exclusively the plexus ana- 

 logous to the lumbar and sacral (w,Jlg. 134). 



The nerve analogous to the phrenic nerve is 

 wanting in Birds, in correspondence with the 

 rudimentary condition of the diaphragm. 



The brachial plexus, formed by the two last 

 cervical and one or two first dorsal nerves, soon 

 becomes blended into a single fasciculus whence 

 all the nerves of the wing are derived. Accord- 

 ing to Cuvier, the first four that are given off 

 are of large size, and are distributed to the 

 great and middle pectoral and subclavian mus- 

 cles. A small filament is then detached which 

 supplies the muscles surrounding the head of 

 the humerus and capsule of the joint ; this re- 

 presents the articular nerve. The rest of the 

 plexus divides into two large nerves, which 

 supply the wing. 



Macartney describes the course of the nerves 

 of the wing in a somewhat different manner, 

 and observes that they more nearly resemble 

 those of the superior extremity in mammalia, 

 than Cuvier has represented. The brachial 

 plexus, according to this author, gives rise to 

 three nerves which are distributed in the follow- 

 ing manner : " The first is a very fine filament, 

 which runs down on the inside of the arm, and 

 is lost about the internal part of the elbow. 

 This is analogous to the internal cutaneous 

 nerve. The second is a large cord ; it gives 

 off a very large branch, which divides into 

 many others, for the supply of the pectoral 

 muscles ; it sends several smaller branches to 

 the muscles under the clavicle and about the 

 joint, and then proceeds to the inner edge of 

 the biceps muscle, along which it descends to 

 the fold of the arm, after giving some large 

 muscular branches. Before it reaches the 

 joint, it divides into two branches; one of 



