The number of _Spinal_ nerves corresponds to that of the vertebræ, through or between which they pass out. Each nerve has two roots, an anterior and posterior, the former of which has no ganglion, and exclusively contains motor elements. The posterior or dorsal has a ganglionic enlargement, and contains sensory elements only. After leaving the 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_. [Illustration: Fig. 46. Brain and anterior portion of the spinal chord of Trigla (Gurnard), showing the globular swellings at the base of the anterior spinal nerves.] A _Sympathic nervous_ system appears to be absent in _Branchiostoma_, and has not yet been clearly made out in _Cyclostomes_. It is well developed in the _Palæichthyes_, but without cephalic portion. This latter is present in all Osseous fishes, in which communication of the Sympathic has been found to exist with all cerebral nerves, except the olfactory, optic, and acustic. The sympathic trunks run along each side of the aorta and the back of the abdomen into the hæmal 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 emerge which are distributed to the various viscera. CHAPTER VII. THE ORGANS OF SENSE. Characteristic of the _Organ of Smell_ in Fishes is that it has no relation whatever to the respiratory function, with the exception of the _Dipnoi_, in which possibly part of the water received for respiration passes through the nasal sac.