THE FROG 89 



perineurium, whose substance is continuous with the epineurium. 

 Yet again, the individual fibres themselves are separated by a small 

 amount of the same kind of tissue, here termed the endoneurium. 

 The neuroglia cells are found all through the central nervous 

 system and are much-branched cells, whose fibrous processes connect 

 up with one another in such a manner as to make a supporting 

 scaffolding for the more delicate nerve cells and processes. 



Sympathetic or Involuntary Nervous System. 



The sympathetic system in Rana is well developed, and 

 consists of a double chain of ganglia joined by longitudinally 

 running strands and closely connected with both the cranial 

 and spinal nerves. The ganglia and cords are usually covered 

 with a layer of black pigment, which makes them easy to follow. 

 It takes origin on each side in the pro-otic ganglion, and runs 

 backwards in the cranium to leave it with the ninth and tenth 

 cranial nerves through the jugular foramen. The first ganglion 

 is formed on the postero-ventral aspect of the hypoglossal nerve, 

 and is connected with this nerve by a few fine fibres. Thence it 

 passes back close to the systemic arch, and then along the sides of 

 the dorsal aorta forming eight or nine ganglia, the number is not 

 constant, while so doing. Each ganglion is connected with its 

 corresponding spinal ganglion by means of a nerve stalk, the ramus 

 communicans, and these increase in length as they pass backwards. 

 The last ganglion is connected to the coccygeal nerve by several fine 

 rami communicantes. The ganglia also give off numerous very 

 fine branches, which pass round the dorsal aorta in the form of a 

 very delicate network or plexus. A similar but more marked rami- 

 fication, the cardiac plexus, is formed on the surface of the heart, 

 where it is connected with two ganglia to be dealt with shortly, and 

 around the roots of the great vessels. Branches from the third, 

 fourth and fifth ganglia on each side pass vent rally to two large 

 ganglia, the coaliac ganglia, situated one on each side of the cceliaco- 

 mesenteric artery, and so form the solar plexus. Branches from this 

 plexus run to form marked plexuses of fibres in the walls of the 

 stomach and intestine, and others in the uro-genital organs and the 

 remaining viscera. In fact, branches of this group of nerves spread 

 to all parts of the body in which involuntary muscle fibres are 

 present, and to the various viscera not directly under the control 

 of the will, and hence the term the involuntary nervous system is 

 quite applicable. 



The first of the two ganglia in the heart referred to above is 

 Remak's ganglion lying in the wall of the sinus venosus, and the 

 second is Bidder's ganglion, situated in the auricular septum near the 



