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. Il, HI, IV); these 
descend with 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, 
Ill, 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 sympathetic 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 em.) 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 trunk 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 
by 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; 
they 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 cceliac 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 ganglia 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 cceliac, 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 hemorrhoidal, 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 trunks are close to the middle sacral artery 
and have each three ganglia when this part is fully developed. The caudal trunks 
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. 
