gg > THE FROG. PHYLUM CHORDATA 



The brain, like the spinal cord, contains both white and grey 

 matter. Most of the grey matter adjoins the ventricles as that of 

 the spinal cord adjoins' the central canal, but the grey layer or 

 cortex which in higher animals overUes the white matter of the 

 pallium (p. 6io) is represented by a rudiment. 



From the brain proceed several pairs of cranial nerves (Fig. 

 302) ; these follow the same general plan in all vertebrates, but 

 they are best seen and studied in the cartilaginous fishes (pp. 338, 

 342). The chief differences from the dogfish shown by the cranial 

 nerves of the frog are these. V (trigeminal) has all four branches 

 well developed. VI (abducent) supplies, as well as the external 

 rectus, the musculus retractor bulbi of the eye, which surrounds 

 the optic nerve and pulls the eyeball back into its socket ; this 

 muscle is not present in most vertebrates. VII (facial) has 

 neither superficial ophthalmic nor buccal rami, and the lateral 

 Hne portion of the external mandibular is also missing ; before 

 leaving the skull the facial is closely associated with the trigeminal 

 in a compound pro-otic ganglion, formed by the fusion of the 

 Gasserian and geniculate gangHa. The pretrematic branch of IX 

 (glossopharyngeal) is absent, and the post-trematic, which con- 

 tains somatic sensory fibres, supplies the tongue and other 

 structures in the mouth ; there is a connection, known as Jacob- 

 son's anastomosis, by visceral sensory fibres from the pharyngeal 

 ramus to the palatine of VII. X (pneumogastric) runs with IX 

 as far as its ganglion ; there is neither pretrematic nor lateral line 

 branch, and the post-trematics are represented by a laryngeal 

 to the muscles of the hyoid and larynx ; the vagus branch supplies 

 the lungs as well as other internal organs. XI (spinal accessory) 

 has a transient existence in the embryo, and XII (hypoglossal) 

 has, by the shortening of the skull, come to lie outside it and 

 appears as the first spinal. 



AUTONOMIC SYSTEM 



The autonomic or sympathetic system possesses a long nerve- 

 cord on each side of the body below the backbone and alongside 

 the aorta and systemic arch. It is connected by a ramus com- 

 municans with each spinal nerve. At the junction of each ramus 

 communicans the sympathetic cord swells into a ganglion. 

 Between the first two ganglia it is double, becoming thus a loop, 

 the annulus of Vieussens or ansa subclavia, through which passes 



