108 FISHES. 



ment, and contains sensory elements only. After leaving tlie 

 vertebral canal each spinal nerve usually divides into a 

 dorsal and ventral branch. The Gadoids show that peculiarity 

 that each of the posterior roots of some or many of the spinal 

 nerves possesses two separate threads, each of which has a 

 ganglion of its own ; the one of these threads joins the dorsal 

 and the other the ventral branch. In fishes in which the 

 spinal chord is very short, as in Plectognaths, Lophius, the 

 roots of the nerves are extremely long, forming a thick 

 Cauda equina. The additional function which the (five) 

 anterior spinal nerves of Trigla have to perform in supplying 

 the sensitive pectoral appendages and their 

 muscles has caused the development of a paired 

 series of globular swellings of the corresponding 

 portion of the spinal chord, A similar structure 

 is found in Polynemus. 



A Si/mpatliic nervous system appears to be 



Fig. 46. absent in Branchiostoma, and has not yet been 



Brain and an- clearly made out in Cydostomcs. It is well 



of the spinal developed in the Palojichthycs, but without 



chord of Trig- ccphahc portiou. This latter is present in all 



? "™^"^' Osseous fishes, in which communication of the 



showing the ' 



globular swei- Sympathic has been found to exist with all 



ings a e ggj-gi^j^-g^j nerves, except the olfactory, optic, and 



anterior spi- acustic. The sympatliic trunks run along each 



nai nerves. g- j^ ^^ ^^^^ aorta and the back of the abdomen 



into the hiemal canal ; communicate in their course with the 



ventral branches of each of the spinal nerves ; and, finally, 



often blend together into a common trunk beneath the 



tail. At the points of communication with the cerebral and 



spinal nerves frequently ganglia are developed, from which 



nerves emero;e which are distributed to the various viscera. 



