222 DISSECTION OF THE DOG 



two nuclei are connected with each other and with the grey matter of the surface 

 of the hemisphere at the olfactory tubercle. 



Claustrum. — The claustrum is a very thin layer of grey matter lateral 

 to the corpus striatum and very nearly co-extensive with the insula, i.e. the 

 convolutions hidden in the Sylvian fissure. 



Capsttla interna. — The term internal capsule is applied to the white 

 matter separating the caudate nucleus and the thalamus on the one side from 

 the lentiform nucleus on the other. The continuity of the capsule is much 

 interfered with anteriorly by the grey strands crossing from one nucleus to 

 the other. In horizontal sections the capsule is bent opposite the stria termi- 

 nalis at what is known as the knee of the internal capsule (genu capsulse 

 internse). The limb in front of the genu may be designated the frontal part 

 (pars frontalis capsulse internae), and the limb behind the genu the occipital 

 part of the capsule (pars occipitalis capsulse internee). 



The internal capsule is of great importance since it contains fibres derived 

 from or passing to various parts of the cerebral cortex. The divergence of 

 the fibres dorsal to the capsule is referred to as the corona radiata. 



Capstjla externa. — The term external capsule is applied to the layer 

 of white matter between the lentiform nucleus and the claustrum. 



Rhombencephalon. — Attention must now be directed to the hind-brain 

 composed of the medulla oblongata, the pons, and the cerebellum. 



Medulla oblongata. — An arbitrary limit to the spinal cord is made 

 on a level with the foramen magnum ; but when the brain and cord 

 have been removed from the body, it is difficult to say exactly where the cord 

 ends and the medulla oblongata begins. The medulla extends as far in a 

 nasal direction as the pons. At first it has the same general form as the spinal 

 cord ; but soon it becomes dorso-ventrally flattened and expanded laterally. 

 The increase in width is associated with the opening out of the central canal 

 (continued from the spinal cord) into the fourth ventricle. 



The bilateral character of the medulla oblongata is made manifest on the 

 surface by the presence of ventral and dorsal median fissures continuous with 

 the like land-marks of the spinal cord. While the ventral median fissure 

 (fissura mediana ventralis) can be traced as far as the pons, where it terminates 

 abruptly at the foramen ccecum, it is not of uniform depth throughout. About 

 the aboral end of the medulla oblongata numerous fibres pass from one side 

 to the other (the decussation of the pyramids) and cause the fissure to be very 

 shallow at this point. The dorsal median fissure (fissura mediana dorsalis) is 

 not so extensive. It is continued only to the extremity of the fourth ventricle, 

 where it apparently widens out in conformity with the lateral divergence of 

 the two halves of the dorsal portion of the medulla oblongata. 



Along each side of the ventral median fissure is a very definite strand of 

 nerve fibres known as the pyramid of the medulla oblongata (pyramis [me- 

 dullas oblongata]) . Followed towards the cerebrum, the pyramid disappears 



