38 Anatomy of- the; Rabbit. 



and a ventral structure of a commissural nature, the pons. The my- 

 elencephalon is a transitional portion connecting the brain with the 

 spinal cord. The cavity of the rhombencephalon is the fourth ventricle. 

 It is a peculiarly shaped space, the floor and lateral walls' of which are 

 very greatly thickened, while the roof is for the most part thin. The 

 roof appears at first sight to be formed largely by the cerebellum, but is 

 in reality formed by two membranes underlying the latter. One of these, 

 the anterior medullary velum, is connected forwards with the mesence- 

 phalon, while the other, the posterior medullary velum, covers a triangular 

 space over which the posterior margin of the cerebellum does not extend. 

 The posterior medullary velum has the same structure as the chorioid 

 plexus of the third ventricle, but is much more poorly developed. 



Apart from its principal divisions, which, as indicated above, are more 

 or less common to all vertebrates, the brain depends for its external form 

 on the elaboration of certain parts in comparison with others. In the 

 mammalia the cerebral hemispheres and the cerebellum are the chief form- 

 determinants, although the pons and the corpora quadrigemina may also 

 be considered in this connection. It will be seen also that the form of 

 the brain is more or less dependent on the existence at certain places of 

 well-marked flexures (cf. Plate II). The first of these, the cephalic 

 flexure, is in the region of the mesencephalon, the anterior portion of the 

 brain being bent downward ; the second, or pontine flexure, is at the fourth 

 •ventricle; while the third, or cervical flexure , is at the point where the 

 myelencephalon passes over into the spinal cord. 



The peripheral nervous system embraces two groups of paired and, 

 for the most part, metamerically arranged nerves, namely, the spinal 

 nerves — those arising from the spinal cord and leaving the vertebral 

 column through the intervertebral foramina; and the cranial or cerebral 

 nerves — those arising from the brain and passing through the foramina 

 of the skull. Of these the spinal nerves are less modified, both in 

 structure and distribution. 



A spinal nerve (Fig. 21) is a product'of two roots, one of which, the 

 radix posterior, is sensory and arises from the dorsal side of the cord, 

 while the other, the radix anterior, is motor and arises from its ventral 

 side. The posterior root bears a root-ganglion. The combined nerve is 

 distributed in three chief parts, of which two, the anterior and posterior 

 rami, are distributed as mixed nerves to the skeletal muscle and the skin, 

 — the anterior ramus being in practically all cases the prominent nerve — 

 while the third, the ramus communicans, is connected with the sym- 

 pathetic trunk. 



The sympathetic trunk is formed on either side by a series of ganglia 

 joined together by connecting cords. It is connected not only with the 

 spinal nerves through the rami communicantes, but also with a series of 

 prevertebral (coeliac, superior mesenteric, etc.) ganglia and plexuses 

 from which nerves pass, chiefly in association with the bloodvessels, to 

 the peripheral ganglia and plexuses of various organs. 



The anterior and posterior rami of the spinal nerves contain both 

 sensory and motor components, and are typical somatic nerves. The 

 ramus communicans is a visceral connection containing similar com- 



