76 DESCRIPTIONS OF PREPARATIONS. 



and auditory nerves have a common origin, and the ganglion of the former unites 

 with the Gasserian ganglion. The ganglion of the glossopharyngeal is fused with 

 that of the vagus, and the hypoglossus is the first spinal nerve. Of the latter there are 

 ten in all. The anterior and posterior nerve-roots unite outside the neural canal. 

 Round the ganglia, formed at the place of union, there lie sacs containing crystals 

 of calcium carbonate the so-called glands of Swammerdam. The Gasserian gan- 

 glion is similarly surrounded. The sympathetic trunk commences at the Gasserian 

 ganglion, is connected with the vagal ganglion, and passes out with the vagus 

 through a foramen in the exoccipital. A ramus communicans connects it with 

 each of the spinal nerves from the first to the sixth inclusive, and a single ganglion, 

 corresponds to each ramus. But the posterior spinal nerves, especially the tenth, 

 give off a variable number of rami communicantes, and the ganglia vary corre- 

 spondingly. The first and second sympathetic ganglia send important (accelerator) 

 twigs to the heart : the fifth sends a twig to the caeliac plexus, which is in connec- 

 tion with the root of the mesenteric artery. 



The sinus venosus of the heart is divided into a large right moiety receiving 

 the two venae cavae superiores and the vena cava inferior, and a small left moiety 

 receiving the pulmonary veins. Two valves guard the aperture into the right 

 auricle. A thin non-muscular septum separates the two auricles : its lower free 

 edge is adherent to the two auriculo-ventricular valves, one anterior, the other 

 posterior. Their free edges and under surfaces are tied by chordae tendineae 

 to the walls of the ventricles. These walls are produced into trabeculae which 

 have a fixed direction and therefore influence the course of the arterial and 

 venous blood-currents respectively. A conus arteriosus leads from the ventricle : 

 its walls are yellowish, semi-transparent, and contain striated muscular fibre. From 

 it springs the truncus aortae which is extremely short and gives origin at once to 

 a right and left branch; its walls are whitish, somewhat opaque, and contain 

 only smooth muscle fibres. The conus is separated from the ventricle by three 

 valves, and from the truncus also by three valves. Of these latter, one is a large 

 right valve prolonged as a spiral fold down the dorsal wall of the conus ; the 

 other two are small valves lying a little to the left, one dorsally, the other 

 ventrally. A vertical septum, placed transversely, divides the origin of the pul- 

 monary arteries from the origins of the aortae and carotids. It is continued into 

 the sinus of the large valve, is fixed to its free edge and to the wall of the conus 

 between the two small valves. The cavity of the conus is consequently divisible 

 physiologically into a dorsal portion which leads to the pulmonary arteries and a 

 ventral from which arise carotids arid aortae. Each half of the truncus contains three 

 vessels, carotid anteriorly, aorta in the middle, and pulmonary artery posteriorly. 

 A fibrous band replaces a vessel, the ductus Botalli or primitive union between the 

 carotid and aorta of each side. At the spot where the carotid splits into the 

 lingual artery, which corresponds as in Reptilia to the external carotid of Aves 

 and Mammalia, and into the carotid so-called in Amphibia, or internal carotid 

 of higher Vertebrata, there is a rete mirabile, the carotid gland, formed by the 

 development of anastomoses between the lingual or external -carotid, and the 

 first branchial artery of the Tadpole (the common stem of the lingual and 

 carotid of the adult) where they are contiguous to one another. The two aortae 

 unite into a sub-vertebral aorta under the backbone, and the left arch gives off a 



