*Gross Structure of Bones.*—The gross structure of the bones is best learned by studying both dry and fresh specimens. (See Practical Work.) The ends of the bones are capped by a layer of smooth, elastic cartilage, while all the remaining surface is covered by a rather dense sheath of connective tissue, called the _periosteum_. Usually the central part of the long bones is hollow, being filled with a fatty substance known as the _yellow marrow_. Around the marrow cavity the bone is very dense and compact, but most of the material forming the ends is porous and spongy. These materials are usually referred to as the _compact substance_ and the _cancellous_, or _spongy, substance_ of the bones (Fig. 93). The arrangement of the compact and spongy substance varies with the different bones. In the short bones (wrist and ankle bones, vertebræ, etc.) and also in the flat bones (skull bones, ribs, shoulder blades, etc.) there is no cavity for the yellow marrow, all of the interior space being filled with the spongy substance. The _red marrow_, relations of which to the red corpuscles of the blood have already been noted (page 27), occupies the minute spaces in the spongy substance. [Fig. 94] Fig. 94—*Cross section of bone showing minute structure.* Magnified. 1. Surface layer of bone. 2. Deeper portion. 3. Haversian canals from which pass the canaliculi. 4. A lacuna. Observe arrangement of lacunæ at surface and in deeper portion. *Minute Structure of Bone.*—A microscopic examination of a thin slice of bone taken from the compact substance shows this to be porous as well as the spongy substance. Two kinds of small channels are found running through it in different directions, known as the Haversian canals and the canaliculi (Fig. 94). These serve the general purpose of distributing nourishment through the bone. The _Haversian canals_ are larger than the canaliculi and contain small nerves and blood vessels, chiefly capillaries (Fig. 95). They extend lengthwise through the bone. The _canaliculi_ are channels for conveying lymph. They pass out from the Haversian canals at right angles, going to all portions of the compact substance except a thin layer at the surface. In the surface layer of the bone the canaliculi are in communication with the periosteum. [Fig. 95] Fig. 95—*Section showing Haversian canal and contents*, highly magnified (after Schäfer). 1. Arterial capillary. 2. Venous capillary. 3. Nerve fibers. 4. Lymph vessel.