160 COMPARATIVE ANATOMY. 



represented by the first and sometimes also the second spinal nerve. 

 Both, to a greater or less degree, take part in forming the brachial 

 plexus. 1 



SYMPATHETIC. 



The sympathetic nervous system is derived from the ganglia of 

 the cranial and spinal nerves, and, as already mentioned, is 

 distributed mainly to the intestinal tract (in the widest sense), 

 the vascular system, and the glandular organs of the body. 



From each spinal ganglion of the embryo, a nerve grows out, 

 which has been already referred to as an "intestinal" nerve (p. 152). 

 After extending a short distance, dorsal to the cardinal vein, each 

 nerve passes into a small, irregularly-shaped mass of nerve-cells, 

 and these ganglia, like those of the spinal nerves, show originally 

 a segmental arrangement. As they become united together by 

 longitudinal commissures, a chain-like paired sympathetic cord 

 is formed (Fig. 104, S, SI to 810). From its ganglia nerves pass 

 off to the above-mentioned systems of organs, and its original 

 connection with the central nervous system persists. 



The sympathetic extends not only along the vertebral column, 

 but passes anteriorly into the skull, where it comes into relations 

 with a series of the cranial nerves similiar to those which it forms 

 further back with the spinal nerves. In the Frog, where it becomes 

 connected with the ganglion of the vagus, a considerable number of 

 its fibres pass together with the latter nerve to the heart, and thus 

 the nerve which is generally known as the vagus stem is in reality 

 vagus plus sympathetic. 



The original segmental character usually disappears later on, 

 and this is especially the case in those regions where marked 

 modifications of the earlier metameric arrangement of the body 

 have taken place, e.g. in the neck. 



In Crocodiles and Birds a part of the sympathetic cord runs 

 within the vertebrarterial canal alongside the vertebral artery, while 

 in all other Vertebrates the whole cord lies along the ventral side 

 of the vertebral column : it is generally situated close to the latter, 

 and overlies the vertebral ends of the ribs. 



BIBLIOGRAPHY. 



A.HLBORN, F. Unters. uberd. Gehirn der Petromyzonten. Zeitsch.f. iviss. Zool. Bd 



XXXIX. 1883. 

 DOHRN, A. Stud. z. Urgesch. d. Wirbelthierk&rp&rs. Mitthl. d. zool. Stat. z. Neapel. 



Bd. III. 1881 und Bd. IV. 1882. 

 ECKER, A. Zur Entw. Gesch. der Furchenund Windungen dor Grosshirnhemispharen 



im Fotus des Menschen. Arch. f. Anthropologic, Bd. III. Die Hirnwindungen 



des Menschen. Braunschweig, 1869 (1883). 

 EHLERS, E. Die Epiphyse am Gehirn der Plagiostomcn. Zeitsch.f. wiss. Zool. Bd. 



XXX, - 



1 Traces of dorsal roots of the hypoglossal have been found in Mammalian embryos. 



