856 THE NERVOUS SYSTEM OF THE DOG 



tibial vessels. It gives twigs to the hock joint and the extensor brevis muscle, and 

 divides into three dorsal metatarsal nerves (Nn. met. dors. II, III, IV); these 

 descend -ndth the corresponding arteries along the intermetatarsal spaces and unite 

 with the dorsal common digital nerves in supplying the dorsal proper digital nerves. 



The tibial nerve divides at the tarsus into two plantar branches. The medial 

 plantar nerve descends along the medial border of the superficial flexor tendon and 

 divides near the middle of the metatarsus into two branches. Of these, the medial 

 branch constitutes the first plantar common digital nerve (N. dig. comm. plant. I). 

 The lateral branch descends on the superficial flexor tendon and divides into three 

 plantar metatarsal nerves (Nn. met. plant. II, III, IV), which unite with the 

 plantar common digital nerves. The lateral plantar nerve passes down between 

 the flexor tendons, sends twigs to the muscles on the plantar surface of the metatar- 

 sus, and divides into three plantar common digital nerves (Nn. dig. comm. plant. II, 

 III, IV). These descend with the deep plantar metatarsal arteries in the second, 

 third, and fourth intermetatarsal spaces, receive the plantar metatarsal nerves, 

 and divide into plantar proper digital nerves, which pass down the opposed surfaces 

 of the second to the fifth digits. 



The S}rmpathetic system presents few special features worthy of mention. 

 The anterior cervical ganglion is situated behind the bulla ossea in close relation 

 to the internal carotid artery and the ganglion nodosum of the vagus. It is fusi- 

 form and is a little less than half an inch (ca. 1 cm.) long in a dog of medium size. 

 It forms connections directly or through the carotid plexus with the last seven 

 cranial and first cervical nerves. The cervical trmik unites with the vagus so in- 

 timately as to form a vago-sympathetic trunk. The posterior cervical ganglion 

 is situated at the point where the sympathetic trunk separates from the vagus 

 (Figs. 613, 614). It is usually distinct from the first thoracic ganglion, which is 

 stellate and lies on the longus colli opposite the first space. The two are connected 

 bj' filaments which cross each side of the brachial artery, forming the ansa sub- 

 clavia. Two or three cardiac nerves proceed from these ganglia on the left side; 

 thejr pass to the lateral surface of the aortic arch, where they ramify and concur 

 with the nerves of the right side in the formation of the cardiac plexus. On the 

 right side one or two cardiac nerves appear to come from the vagus and one from 

 the first thoracic ganglion. They ramify on the right side of the trachea and with 

 those of the right side form the cardiac plexus at the base of the heart. From the 

 plexus branches go to the heart, pericardium, and pulmonary artery. The greater 

 splanchnic nerve separates from the posterior part of the thoracic trunk, most often 

 from the twelfth ganglion. It passes into the abdomen between the diaphragm 

 and the psoas minor and joins a small adrenal ganglion close to the coeliac ganglion. 

 The lesser splanchnic nerve arises from the last thoracic and first lumbar ganglia. 

 It may be divided into two or three strands which go to the adrenal plexus. 



The coeliac and anterior mesenteric gangUa are separate. The former is 

 elongated and lies along the posterior face of the origin of the coeliac artery. The 

 latter is smaller and rounded and is in contact with the origin of the anterior mesen- 

 teric artery. On the left side the two ganglia are connected by a trunk which is 

 ganglionic in character. The coeliac, anterior mesenteric, and subsidiary plexuses 

 enlace the corresponding arteries. There is a small elongated posterior mesenteric 

 ganglion in relation to the origin of the posterior mesenteric artery. The posterior 

 mesenteric plexus includes left colic, anterior haemorrhoidal, and spermatic plexuses. 

 The lumbar trunk is deeply placed at the medial side of the psoas minor. It has 

 seven ganglia, which are very small, with the exception of the last. There is a well- 

 developed aortic plexus. The sacral tnanks are close to the middle sacral artery 

 and have each three ganglia when this part is fully developed. The caudal trimks 

 lie on each side of the coccygeal artery; the number and arrangement of the visible 

 ganglia are variable. The pelvic plexus is well developed and contains minute 

 ganglia. 



